I am reading that the volatile keyword is not suitable for thread synchronisation and in fact it is not needed for these purposes at all.
While I understand that using this keyword is not sufficient, I fail to understand why is it completely unnecessary.
For example, assume we have two threads, thread A that only reads from a shared variable and thread B that only writes to a shared variable. Proper synchronisation by e.g. pthreads mutexes is enforced.
IIUC, without the volatile keyword, the compiler may look at the code of thread A and say: “The variable doesn’t appear to be modified here, but we have lots of reads; let’s read it only once, cache the value and optimise away all subsequent reads.” Also it may look at the code of thread B and say: “We have lots of writes to this variable here, but no reads; so, the written values are not needed and thus let’s optimise away all writes.“
Both optimisations would be incorrect. And both one would be prevented by volatile. So, I would likely come to the conclusion that while volatile is not enough to synchronise threads, it is still necessary for any variable shared between threads. (note: I now read that actually it is not required for volatile to prevent write elisions; so I am out of ideas how to prevent such incorrect optimisations)
I understand that I am wrong in here. But why?
For example, assume we have two threads, thread A that only reads from a shared variable and thread B that only writes to a shared variable. Proper synchronisation by e.g. pthreads mutexes is enforced.
IIUC, without the volatile keyword, the compiler may look at the code of thread A and say: “The variable doesn’t appear to be modified here, but we have lots of reads; let’s read it only once, cache the value and optimise away all subsequent reads.” Also it may look at the code of thread B and say: “We have lots of writes to this variable here, but no reads; so, the written values are not needed and thus let’s optimise away all writes.“
Like most thread synchronization primitives, pthreads mutex operations have explicitly defined memory visibility semantics.
Either the platform supports pthreads or it doesn't. If it supports pthreads, it supports pthreads mutexes. Either those optimizations are safe or they aren't. If they're safe, there's no problem. If they're unsafe, then any platform that makes them doesn't support pthreads mutexes.
For example, you say "The variable doesn’t appear to be modified here", but it does -- another thread could modify it there. Unless the compiler can prove its optimization can't break any conforming program, it can't make it. And a conforming program can modify the variable in another thread. Either the compiler supports POSIX threads or it doesn't.
As it happens, most of this happens automatically on most platforms. The compiler is just prevented from having any idea what the mutex operations do internally. Anything another thread could do, the mutex operations themselves could do. So the compiler has to "synchronize" memory before entering and exiting those functions anyway. It can't, for example, keep a value in a register across the call to pthread_mutex_lock because for all it knows, pthread_mutex_lock accesses that value in memory. Alternatively, if the compiler has special knowledge about the mutex functions, that would include knowing about the invalidity of caching values accessible to other threads across those calls.
A platform that requires volatile would be pretty much unusable. You'd need versions of every function or class for the specific cases where an object might be made visible to, or was made visible from, another thread. In many cases, you'd pretty much just have to make everything volatile and not caching values in registers is a performance non-starter.
As you've probably heard many times, volatile's semantics as specified in the C language just do not mix usefully with threads. Not only is it not sufficient, it disables many perfectly safe and nearly essential optimizations.
Shortening the answer already given, you do not need to use volatile with mutexes for a simple reason:
If compiler knows what mutex operations are (by recognizing pthread_* functions or because you used std::mutex), it well knows how to handle access in regards to optimization (which is even required for std::mutex)
If compiler does not recognize them, pthread_* functions are completely opaque to it, and no optimizations involving any sort of non-local duration objects can go across opaque functions
Making the answer even shorter, not using either a mutex or a semaphore, is a bug. As soon as thread B releases the mutex (and thread A gets it), any value in the register which contains the shared variable's value from thread B are guaranteed to be written to cache or memory that will prevent a race condition when thread A runs and reads this variable.
The implementation to guarantee this is architecture/compiler dependent.
The keyword volatile tells the compiler to treat any write or read of the variable as an "observable side-effect." That is all it does. Observable side-effects of course must not be optimized away, and must appear to the outside world as occurring in the order the program indicates; The compiler may not re-order observable side-effects with regard to each other. The compiler is however free to reorder them with respect to non-observables. Therefore, volatile is only appropriate for accessing memory-mapped hardware, Unix-style signal handlers and the like. For inter-thread concurrence, use std::atomic or higher level synchronization objects like mutex, condition_variable, and promise/future.
Related
I'm running a few threads that basically are all returning the same object as a result. Then I wait for all of them to complete, and basically read the results. To avoid needing synchronization, I figured I could just pre-allocate all the result objects in an array or vector and give the threads a pointer to each. At a high level, the code is something like this (simplified):
std::vector<Foo> results(2);
RunThread1(&results[0]);
RunThread2(&results[1]);
WaitForAll();
// Read results
cout << results[0].name << results[1].name;
Basically I'd like to know if there's anything potentially unsafe about this "code". One thing I was wondering is whether the vector should declared volatile such that the reads at the end aren't optimized and output an incorrect value.
The short answer to your question is no, the array should not be declared volatile. For two simple reasons:
Using volatile is not necessary. Every sane multithreading platform provides synchronization primitives with well-defined semantics. If you use them, you don't need volatile.
Using volatile isn't sufficient. Since volatile doesn't have defined multithread semantics on any platform you are likely to use, it alone is not enough to provide synchronization.
Most likely, whatever you do in WaitForAll will be sufficient. For example, if it uses an event, mutex, condition variable, or almost anything like that, it will have defined multithreading semantics that are sufficient to make this safe.
Update: "Just out curiosity, what would be an example of something that happens in the WaitForAll that guarantees safety of the read? Wouldn't it need to effectively tell the compiler somehow to "flush" the cache or avoid optimizations of subsequent read operations?"
Well, if you're using pthreads, then if it uses pthread_join that would be an example that guarantees safety of the read because the documentation says that anything the thread does is visible to a thread that joins it after pthread_join returns.
How it does it is an implementation detail. In practice, on modern systems, there are no caches to flush nor are there any optimizations of subsequent reads that are possible but need to be avoided.
Consider if somewhere deep inside WaitForAll, there's a call to pthread_join. Generally, you simply don't let the compiler see into the internals of pthread_join and thus the compiler has to assume that pthread_join might do anything that another thread could do. So keeping information that another thread might modify in a register across a call to pthread_join would be illegal because pthread_join itself might access or modify that data.
I was wondering is whether the vector should declared volatile such that the reads at the end aren't optimized and output an incorrect value.
No. If there was problem with lack of synchronisation, then volatile would not help.
But there is no problem with lack of synchronisation, since according to your description, you don't access the same object from multiple threads - until you've waited for the threads to complete, which is something that synchronises the threads.
There is a potential problem that if the objects are small (less than about 64 bytes; depending on CPU architecture), then the objects in the array share a cache "line", and access to them may become effectively synchronised due to write contention. This is only a problem if the threads write to the variable a lot in relation to operations that don't access the output object.
It depends on what's in WaitForAll(). If it's a proper synchronization, all is good. Mutexes, for example, or thread join, will result in the proper memory synchronization being put in.
Volatile would not help. It may prevent compiler optimizations, but would not affect anything happening at the CPU level, like caches not being updated. Use proper synchronization, like mutexes, thread join, and then the result will be valid (sequentially coherent). Don't count on silver bullet volatile. Compilers and CPUs are now complex enough that it won't be guaranteed.
Other answers will elaborate on the memory fences and other instructions that the synchronization will put in. :-)
From what I know, the compiler never optimizes a variable that is declared as volatile. However, I have an array declared like this.
volatile long array[8];
And different threads read and write to it. An element of the array is only modified by one of the threads and read by any other thread. However, in certain situations I've noticed that even if I modify an element from a thread, the thread reading it does not notice the change. It keeps on reading the same old value, as if compiler has cached it somewhere. But compiler in principal should not cache a volatile variable, right? So how come this is happening.
NOTE: I am not using volatile for thread synchronization, so please stop giving me answers such as use a lock or an atomic variable. I know the difference between volatile, atomic variables and mutexes. Also note that the architecture is x86 which has proactive cache coherence. Also I read the variable for long enough after it is supposedly modified by the other thread. Even after a long time, the reading thread can't see the modified value.
But compiler in principal should not cache a volatile variable, right?
No, the compiler in principle must read/write the address of the variable each time you read/write the variable.
[Edit: At least, it must do so up to the point at which the the implementation believes that the value at that address is "observable". As Dietmar points out in his answer, an implementation might declare that normal memory "cannot be observed". This would come as a surprise to people using debuggers, mprotect, or other stuff outside the scope of the standard, but it could conform in principle.]
In C++03, which does not consider threads at all, it is up to the implementation to define what "accessing the address" means when running in a thread. Details like this are called the "memory model". Pthreads, for example, allows per-thread caching of the whole of memory, including volatile variables. IIRC, MSVC provides a guarantee that volatile variables of suitable size are atomic, and it will avoid caching (rather, it will flush as far as a single coherent cache for all cores). The reason it provides that guarantee is because it's reasonably cheap to do so on Intel -- Windows only really cares about Intel-based architectures, whereas Posix concerns itself with more exotic stuff.
C++11 defines a memory model for threading, and it says that this is a data race (i.e. that volatile does not ensure that a read in one thread is sequenced relative to a write in another thread). Two accesses can be sequenced in a particular order, sequenced in unspecified order (the standard might say "indeterminate order", I can't remember), or not sequenced at all. Not sequenced at all is bad -- if either of two unsequenced accesses is a write then behavior is undefined.
The key here is the implied "and then" in "I modify an element from a thread AND THEN the thread reading it does not notice the change". You're assuming that the operations are sequenced, but they're not. As far as the reading thread is concerned, unless you use some kind of synchronization the write in the other thread hasn't necessarily happened yet. And actually it's worse than that -- you might think from what I just wrote that it's only the order of operations that is unspecified, but actually the behavior of a program with a data race is undefined.
C
What volatile does:
Guarantees an up-to-date value in the variable, if the variable is modified from an external source (a hardware register, an interrupt, a different thread, a callback function etc).
Blocks all optimizations of read/write access to the variable.
Prevent dangerous optimization bugs that can happen to variables shared between several threads/interrupts/callback functions, when the compiler does not realize that the thread/interrupt/callback is called by the program. (This is particularly common among various questionable embedded system compilers, and when you get this bug it is very hard to track down.)
What volatile does not:
It does not guarantee atomic access or any form of thread-safety.
It cannot be used instead of a mutex/semaphore/guard/critical section. It cannot be used for thread synchronization.
What volatile may or may not do:
It may or may not be implemented by the compiler to provide a memory barrier, to protect against instruction cache/instruction pipe/instruction re-ordering issues in a multi-core environment. You should never assume that volatile does this for you, unless the compiler documentation explicitly states that it does.
With volatile you can only impose that a variable is re-read whenever you use its value. It doesn't guarantee that the different values/representations that are present on different levels of your architecture are consistent.
To have such gurantees you'd need the new utilities from C11 and C++1 concerning atomic access and memory barriers. Many compilers implement these already in terms of extension. E.g the gcc family (clang, icc, etc) have builtins starting with prefix __sync to implement these.
Volatile Keyword only guarantees that the compiler will not use register for this variable. Thus every access to this variable will go and read the memory location. Now, I assume that you have cache coherence among the multiple processors in your architecture. So if one processor writes and other reads it, then it should be visible under normal conditions. However, you should consider the corner cases. Suppose the variable is in the pipeline of one processor core and other processor is trying to read it assuming that has been written, then there is a problem. So essentially, the shared variables should be either guarded by locks or should be protected by using barrier mechanism correctly.
The semantics of volatile are implementation-defined. If a compiler knew that interrupts would be disabled while certain piece of code was executed, and knew that on the target platform there would be no means other than interrupt handlers via which operations on certain storage would be observable, it could register-cache volatile-qualified variables within such storage just the same as it could cache ordinary variables, provided it documented such behavior.
Note that what aspects of behavior are counted as "observable" may be defined in some measure by the implementation. If an implementation documents that it is not intended for use on hardware which uses main RAM accesses to trigger required externally-visible actions, then accesses to main RAM would not be "observable" on that implementation. The implementation would be compatible with hardware which was capable of physically observing such accesses, if nothing cared whether any such accesses were actually seen. If such accesses were required, however, as they would be if the accesses were regarded as "observable", however, the compiler would not be claiming compatibility and would thus make no promise about anything.
For C++:
From what I know, the compiler never optimizes a variable that is declared as volatile.
Your premise is wrong. volatile is a hint to the compiler and doesn't actually guarantee anything. Compilers can choose to prevent some optimizations on volatile variables, but that's it.
volatile isn't a lock, don't try to use it as such.
7.1.5.1
7) [ Note: volatile is a hint to the implementation to avoid
aggressive optimization involving the object because the value of the
object might be changed by means undetectable by an implementation.
See 1.9 for detailed semantics. In general, the semantics of volatile
are intended to be the same in C++ as they are in C. —end note]
Volatile only affects the variable it is in front of. Here in your example, a pointer. Your code: volatile long array[8], the pointer to the first element of the array is volatile, not it's content. (same for objects of any kind)
you could adapt it like in
How do I declare an array created using malloc to be volatile in c++
The volatile keyword has nothing to do with concurrency in C++ at all! It is used to have the compiler prevented from making use of the previous value, i.e., the compiler will generate code accessing a volatile value every time is accessed in the code. The main purpose are things like memory mapped I/O. However, use of volatile has no affect on what the CPU does when reading normal memory: If the CPU has no reason to believe that the value changed in memory, e.g., because there is no synchronization directive, it can just use the value from its cache. To communicate between threads you need some synchronization, e.g., an std::atomic<T>, lock a std::mutex, etc.
C++ accesses by volatile lvalues and C accesses to volatile objects are "abstractly" "observable"--although in practice C behaviour is per the C++ standard not the C standard. Informally, the volatile declaration tells every thread the value might change somehow regardless of the text in any thread. Under the Standards with threads, there isn't any notion of a write by another thread causing a change in an object, volatile or not, shared or not, except for a shared variable by the synchronizing function call at the start of a synchronized critical region. volatile is irrelevant to thread shared objects.
If your code doesn't properly synchronize the thread you are talking about, your one thread reading what an other thread wrote has undefined behaviour. So the compiler can generate any code it wants. If your code is properly synchronized then writes by other threads only happen at thread synchronization calls; you don't need volatile for that.
PS
The standards say "What constitutes an access to an object that
has volatile-qualified type is implementation-defined." So you can't just assume that there is a read access for every dereferencing of a volatile lvalue or a write acces for every assignment through one.
Moreover how ("abstract") "observable" volatile accesses are "actually" manifested is implementation defined. So a compiler might not generate code for hardware accesses corresponding to the defined abstract accesses. Eg maybe only objects with static storage duration and external linkage compiled with a certain flag for linking into special hardware locations can ever be changed from outside the program text, so that other objects' volatile is ignored.
However, in certain situations I've noticed that even if I modify an
element from a thread, the thread reading it does not notice the
change. It keeps on reading the same old value, as if compiler has
cached it somewhere.
This is not because the compiler cached it somewhere, but because the reading thread reads from its CPU core's cache, which might be different from the writing thread's one. To ensure value change propagation across CPU cores, you need to use proper memory fences, and you neither can nor need to use volatile for that in C++.
A coworker and I write software for a variety of platforms running on x86, x64, Itanium, PowerPC, and other 10 year old server CPUs.
We just had a discussion about whether mutex functions such as pthread_mutex_lock() ... pthread_mutex_unlock() are sufficient by themselves, or whether the protected variable needs to be volatile.
int foo::bar()
{
//...
//code which may or may not access _protected.
pthread_mutex_lock(m);
int ret = _protected;
pthread_mutex_unlock(m);
return ret;
}
My concern is caching. Could the compiler place a copy of _protected on the stack or in a register, and use that stale value in the assignment? If not, what prevents that from happening? Are variations of this pattern vulnerable?
I presume that the compiler doesn't actually understand that pthread_mutex_lock() is a special function, so are we just protected by sequence points?
Thanks greatly.
Update: Alright, I can see a trend with answers explaining why volatile is bad. I respect those answers, but articles on that subject are easy to find online. What I can't find online, and the reason I'm asking this question, is how I'm protected without volatile. If the above code is correct, how is it invulnerable to caching issues?
Simplest answer is volatile is not needed for multi-threading at all.
The long answer is that sequence points like critical sections are platform dependent as is whatever threading solution you're using so most of your thread safety is also platform dependent.
C++0x has a concept of threads and thread safety but the current standard does not and therefore volatile is sometimes misidentified as something to prevent reordering of operations and memory access for multi-threading programming when it was never intended and can't be reliably used that way.
The only thing volatile should be used for in C++ is to allow access to memory mapped devices, allow uses of variables between setjmp and longjmp, and to allow uses of sig_atomic_t variables in signal handlers. The keyword itself does not make a variable atomic.
Good news in C++0x we will have the STL construct std::atomic which can be used to guarantee atomic operations and thread safe constructs for variables. Until your compiler of choice supports it you may need to turn to the boost library or bust out some assembly code to create your own objects to provide atomic variables.
P.S. A lot of the confusion is caused by Java and .NET actually enforcing multi-threaded semantics with the keyword volatile C++ however follows suit with C where this is not the case.
Your threading library should include the apropriate CPU and compiler barriers on mutex lock and unlock. For GCC, a memory clobber on an asm statement acts as a compiler barrier.
Actually, there are two things that protect your code from (compiler) caching:
You are calling a non-pure external function (pthread_mutex_*()), which means that the compiler doesn't know that that function doesn't modify your global variables, so it has to reload them.
As I said, pthread_mutex_*() includes a compiler barrier, e.g: on glibc/x86 pthread_mutex_lock() ends up calling the macro lll_lock(), which has a memory clobber, forcing the compiler to reload variables.
If the above code is correct, how is it invulnerable to caching
issues?
Until C++0x, it is not. And it is not specified in C. So, it really depends on the compiler. In general, if the compiler does not guarantee that it will respect ordering constraints on memory accesses for functions or operations that involve multiple threads, you will not be able to write multithreaded safe code with that compiler. See Hans J Boehm's Threads Cannot be Implemented as a Library.
As for what abstractions your compiler should support for thread safe code, the wikipedia entry on Memory Barriers is a pretty good starting point.
(As for why people suggested volatile, some compilers treat volatile as a memory barrier for the compiler. It's definitely not standard.)
The volatile keyword is a hint to the compiler that the variable might change outside of program logic, such as a memory-mapped hardware register that could change as part of an interrupt service routine. This prevents the compiler from assuming a cached value is always correct and would normally force a memory read to retrieve the value. This usage pre-dates threading by a couple decades or so. I've seen it used with variables manipulated by signals as well, but I'm not sure that usage was correct.
Variables guarded by mutexes are guaranteed to be correct when read or written by different threads. The threading API is required to ensure that such views of variables are consistent. This access is all part of your program logic and the volatile keyword is irrelevant here.
With the exception of the simplest spin lock algorithm, mutex code is quite involved: a good optimized mutex lock/unlock code contains the kind of code even excellent programmer struggle to understand. It uses special compare and set instructions, manages not only the unlocked/locked state but also the wait queue, optionally uses system calls to go into a wait state (for lock) or wake up other threads (for unlock).
There is no way the average compiler can decode and "understand" all that complex code (again, with the exception of the simple spin lock) no matter way, so even for a compiler not aware of what a mutex is, and how it relates to synchronization, there is no way in practice a compiler could optimize anything around such code.
That's if the code was "inline", or available for analyse for the purpose of cross module optimization, or if global optimization is available.
I presume that the compiler doesn't actually understand that
pthread_mutex_lock() is a special function, so are we just protected
by sequence points?
The compiler does not know what it does, so does not try to optimize around it.
How is it "special"? It's opaque and treated as such. It is not special among opaque functions.
There is no semantic difference with an arbitrary opaque function that can access any other object.
My concern is caching. Could the compiler place a copy of _protected
on the stack or in a register, and use that stale value in the
assignment?
Yes, in code that act on objects transparently and directly, by using the variable name or pointers in a way that the compiler can follow. Not in code that might use arbitrary pointers to indirectly use variables.
So yes between calls to opaque functions. Not across.
And also for variables which can only be used in the function, by name: for local variables that don't have either their address taken or a reference bound to them (such that the compiler cannot follow all further uses). These can indeed be "cached" across arbitrary calls include lock/unlock.
If not, what prevents that from happening? Are variations of this
pattern vulnerable?
Opacity of the functions. Non inlining. Assembly code. System calls. Code complexity. Everything that make compilers bail out and think "that's complicated stuff just make calls to it".
The default position of a compiler is always the "let's execute stupidly I don't understand what is being done anyway" not "I will optimize that/let's rewrite the algorithm I know better". Most code is not optimized in complex non local way.
Now let's assume the absolute worse (from out point of view which is that the compiler should give up, that is the absolute best from the point of view of an optimizing algorithm):
the function is "inline" (= available for inlining) (or global optimization kicks in, or all functions are morally "inline");
no memory barrier is needed (as in a mono-processor time sharing system, and in a multi-processor strongly ordered system) in that synchronization primitive (lock or unlock) so it contains no such thing;
there is no special instruction (like compare and set) used (for example for a spin lock, the unlock operation is a simple write);
there is no system call to pause or wake threads (not needed in a spin lock);
then we might have a problem as the compiler could optimize around the function call. This is fixed trivially by inserting a compiler barrier such as an empty asm statement with a "clobber" for other accessible variables. That means that compiler just assumes that anything that might be accessible to a called function is "clobbered".
or whether the protected variable needs to be volatile.
You can make it volatile for the usual reason you make things volatile: to be certain to be able to access the variable in the debugger, to prevent a floating point variable from having the wrong datatype at runtime, etc.
Making it volatile would actually not even fix the issue described above as volatile is essentially a memory operation in the abstract machine that has the semantics of an I/O operation and as such is only ordered with respect to
real I/O like iostream
system calls
other volatile operations
asm memory clobbers (but then no memory side effect is reordered around those)
calls to external functions (as they might do one the above)
Volatile is not ordered with respect to non volatile memory side effects. That makes volatile practically useless (useless for practical uses) for writing thread safe code in even the most specific case where volatile would a priori help, the case where no memory fence is ever needed: when programming threading primitives on a time sharing system on a single CPU. (That may be one of the least understood aspects of either C or C++.)
So while volatile does prevent "caching", volatile doesn't even prevent compiler reordering of lock/unlock operation unless all shared variables are volatile.
Locks/synchronisation primitives make sure the data is not cached in registers/cpu cache, that means data propagates to memory. If two threads are accessing/ modifying data with in locks, it is guaranteed that data is read from memory and written to memory. We don't need volatile in this use case.
But the case where you have code with double checks, compiler can optimise the code and remove redundant code, to prevent that we need volatile.
Example: see singleton pattern example
https://en.m.wikipedia.org/wiki/Singleton_pattern#Lazy_initialization
Why do some one write this kind of code?
Ans: There is a performance benefit of not accuiring lock.
PS: This is my first post on stack overflow.
Not if the object you're locking is volatile, eg: if the value it represents depends on something foreign to the program (hardware state).
volatile should NOT be used to denote any kind of behavior that is the result of executing the program.
If it's actually volatile what I personally would do is locking the value of the pointer/address, instead of the underlying object.
eg:
volatile int i = 0;
// ... Later in a thread
// ... Code that may not access anything without a lock
std::uintptr_t ptr_to_lock = &i;
some_lock(ptr_to_lock);
// use i
release_some_lock(ptr_to_lock);
Please note that it only works if ALL the code ever using the object in a thread locks the same address. So be mindful of that when using threads with some variable that is part of an API.
As demonstrated in this answer I recently posted, I seem to be confused about the utility (or lack thereof) of volatile in multi-threaded programming contexts.
My understanding is this: any time a variable may be changed outside the flow of control of a piece of code accessing it, that variable should be declared to be volatile. Signal handlers, I/O registers, and variables modified by another thread all constitute such situations.
So, if you have a global int foo, and foo is read by one thread and set atomically by another thread (probably using an appropriate machine instruction), the reading thread sees this situation in the same way it sees a variable tweaked by a signal handler or modified by an external hardware condition and thus foo should be declared volatile (or, for multithreaded situations, accessed with memory-fenced load, which is probably a better a solution).
How and where am I wrong?
The problem with volatile in a multithreaded context is that it doesn't provide all the guarantees we need. It does have a few properties we need, but not all of them, so we can't rely on volatile alone.
However, the primitives we'd have to use for the remaining properties also provide the ones that volatile does, so it is effectively unnecessary.
For thread-safe accesses to shared data, we need a guarantee that:
the read/write actually happens (that the compiler won't just store the value in a register instead and defer updating main memory until much later)
that no reordering takes place. Assume that we use a volatile variable as a flag to indicate whether or not some data is ready to be read. In our code, we simply set the flag after preparing the data, so all looks fine. But what if the instructions are reordered so the flag is set first?
volatile does guarantee the first point. It also guarantees that no reordering occurs between different volatile reads/writes. All volatile memory accesses will occur in the order in which they're specified. That is all we need for what volatile is intended for: manipulating I/O registers or memory-mapped hardware, but it doesn't help us in multithreaded code where the volatile object is often only used to synchronize access to non-volatile data. Those accesses can still be reordered relative to the volatile ones.
The solution to preventing reordering is to use a memory barrier, which indicates both to the compiler and the CPU that no memory access may be reordered across this point. Placing such barriers around our volatile variable access ensures that even non-volatile accesses won't be reordered across the volatile one, allowing us to write thread-safe code.
However, memory barriers also ensure that all pending reads/writes are executed when the barrier is reached, so it effectively gives us everything we need by itself, making volatile unnecessary. We can just remove the volatile qualifier entirely.
Since C++11, atomic variables (std::atomic<T>) give us all of the relevant guarantees.
You might also consider this from the Linux Kernel Documentation.
C programmers have often taken volatile to mean that the variable
could be changed outside of the current thread of execution; as a
result, they are sometimes tempted to use it in kernel code when
shared data structures are being used. In other words, they have been
known to treat volatile types as a sort of easy atomic variable, which
they are not. The use of volatile in kernel code is almost never
correct; this document describes why.
The key point to understand with regard to volatile is that its
purpose is to suppress optimization, which is almost never what one
really wants to do. In the kernel, one must protect shared data
structures against unwanted concurrent access, which is very much a
different task. The process of protecting against unwanted
concurrency will also avoid almost all optimization-related problems
in a more efficient way.
Like volatile, the kernel primitives which make concurrent access to
data safe (spinlocks, mutexes, memory barriers, etc.) are designed to
prevent unwanted optimization. If they are being used properly, there
will be no need to use volatile as well. If volatile is still
necessary, there is almost certainly a bug in the code somewhere. In
properly-written kernel code, volatile can only serve to slow things
down.
Consider a typical block of kernel code:
spin_lock(&the_lock);
do_something_on(&shared_data);
do_something_else_with(&shared_data);
spin_unlock(&the_lock);
If all the code follows the locking rules, the value of shared_data
cannot change unexpectedly while the_lock is held. Any other code
which might want to play with that data will be waiting on the lock.
The spinlock primitives act as memory barriers - they are explicitly
written to do so - meaning that data accesses will not be optimized
across them. So the compiler might think it knows what will be in
shared_data, but the spin_lock() call, since it acts as a memory
barrier, will force it to forget anything it knows. There will be no
optimization problems with accesses to that data.
If shared_data were declared volatile, the locking would still be
necessary. But the compiler would also be prevented from optimizing
access to shared_data within the critical section, when we know that
nobody else can be working with it. While the lock is held,
shared_data is not volatile. When dealing with shared data, proper
locking makes volatile unnecessary - and potentially harmful.
The volatile storage class was originally meant for memory-mapped I/O
registers. Within the kernel, register accesses, too, should be
protected by locks, but one also does not want the compiler
"optimizing" register accesses within a critical section. But, within
the kernel, I/O memory accesses are always done through accessor
functions; accessing I/O memory directly through pointers is frowned
upon and does not work on all architectures. Those accessors are
written to prevent unwanted optimization, so, once again, volatile is
unnecessary.
Another situation where one might be tempted to use volatile is when
the processor is busy-waiting on the value of a variable. The right
way to perform a busy wait is:
while (my_variable != what_i_want)
cpu_relax();
The cpu_relax() call can lower CPU power consumption or yield to a
hyperthreaded twin processor; it also happens to serve as a memory
barrier, so, once again, volatile is unnecessary. Of course,
busy-waiting is generally an anti-social act to begin with.
There are still a few rare situations where volatile makes sense in
the kernel:
The above-mentioned accessor functions might use volatile on
architectures where direct I/O memory access does work. Essentially,
each accessor call becomes a little critical section on its own and
ensures that the access happens as expected by the programmer.
Inline assembly code which changes memory, but which has no other
visible side effects, risks being deleted by GCC. Adding the volatile
keyword to asm statements will prevent this removal.
The jiffies variable is special in that it can have a different value
every time it is referenced, but it can be read without any special
locking. So jiffies can be volatile, but the addition of other
variables of this type is strongly frowned upon. Jiffies is considered
to be a "stupid legacy" issue (Linus's words) in this regard; fixing it
would be more trouble than it is worth.
Pointers to data structures in coherent memory which might be modified
by I/O devices can, sometimes, legitimately be volatile. A ring buffer
used by a network adapter, where that adapter changes pointers to
indicate which descriptors have been processed, is an example of this
type of situation.
For most code, none of the above justifications for volatile apply.
As a result, the use of volatile is likely to be seen as a bug and
will bring additional scrutiny to the code. Developers who are
tempted to use volatile should take a step back and think about what
they are truly trying to accomplish.
I don't think you're wrong -- volatile is necessary to guarantee that thread A will see the value change, if the value is changed by something other than thread A. As I understand it, volatile is basically a way to tell the compiler "don't cache this variable in a register, instead be sure to always read/write it from RAM memory on every access".
The confusion is because volatile isn't sufficient for implementing a number of things. In particular, modern systems use multiple levels of caching, modern multi-core CPUs do some fancy optimizations at run-time, and modern compilers do some fancy optimizations at compile time, and these all can result in various side effects showing up in a different order from the order you would expect if you just looked at the source code.
So volatile is fine, as long as you keep in mind that the 'observed' changes in the volatile variable may not occur at the exact time you think they will. Specifically, don't try to use volatile variables as a way to synchronize or order operations across threads, because it won't work reliably.
Personally, my main (only?) use for the volatile flag is as a "pleaseGoAwayNow" boolean. If I have a worker thread that loops continuously, I'll have it check the volatile boolean on each iteration of the loop, and exit if the boolean is ever true. The main thread can then safely clean up the worker thread by setting the boolean to true, and then calling pthread_join() to wait until the worker thread is gone.
volatile is useful (albeit insufficient) for implementing the basic construct of a spinlock mutex, but once you have that (or something superior), you don't need another volatile.
The typical way of multithreaded programming is not to protect every shared variable at the machine level, but rather to introduce guard variables which guide program flow. Instead of volatile bool my_shared_flag; you should have
pthread_mutex_t flag_guard_mutex; // contains something volatile
bool my_shared_flag;
Not only does this encapsulate the "hard part," it's fundamentally necessary: C does not include atomic operations necessary to implement a mutex; it only has volatile to make extra guarantees about ordinary operations.
Now you have something like this:
pthread_mutex_lock( &flag_guard_mutex );
my_local_state = my_shared_flag; // critical section
pthread_mutex_unlock( &flag_guard_mutex );
pthread_mutex_lock( &flag_guard_mutex ); // may alter my_shared_flag
my_shared_flag = ! my_shared_flag; // critical section
pthread_mutex_unlock( &flag_guard_mutex );
my_shared_flag does not need to be volatile, despite being uncacheable, because
Another thread has access to it.
Meaning a reference to it must have been taken sometime (with the & operator).
(Or a reference was taken to a containing structure)
pthread_mutex_lock is a library function.
Meaning the compiler can't tell if pthread_mutex_lock somehow acquires that reference.
Meaning the compiler must assume that pthread_mutex_lock modifes the shared flag!
So the variable must be reloaded from memory. volatile, while meaningful in this context, is extraneous.
Your understanding really is wrong.
The property, that the volatile variables have, is "reads from and writes to this variable are part of perceivable behaviour of the program". That means this program works (given appropriate hardware):
int volatile* reg=IO_MAPPED_REGISTER_ADDRESS;
*reg=1; // turn the fuel on
*reg=2; // ignition
*reg=3; // release
int x=*reg; // fire missiles
The problem is, this is not the property we want from thread-safe anything.
For example, a thread-safe counter would be just (linux-kernel-like code, don't know the c++0x equivalent):
atomic_t counter;
...
atomic_inc(&counter);
This is atomic, without a memory barrier. You should add them if necessary. Adding volatile would probably not help, because it wouldn't relate the access to the nearby code (eg. to appending of an element to the list the counter is counting). Certainly, you don't need to see the counter incremented outside your program, and optimisations are still desirable, eg.
atomic_inc(&counter);
atomic_inc(&counter);
can still be optimised to
atomically {
counter+=2;
}
if the optimizer is smart enough (it doesn't change the semantics of the code).
For your data to be consistent in a concurrent environment you need two conditions to apply:
1) Atomicity i.e if I read or write some data to memory then that data gets read/written in one pass and cannot be interrupted or contended due to e.g a context switch
2) Consistency i.e the order of read/write ops must be seen to be the same between multiple concurrent environments - be that threads, machines etc
volatile fits neither of the above - or more particularly, the c or c++ standard as to how volatile should behave includes neither of the above.
It's even worse in practice as some compilers ( such as the intel Itanium compiler ) do attempt to implement some element of concurrent access safe behaviour ( i.e by ensuring memory fences ) however there is no consistency across compiler implementations and moreover the standard does not require this of the implementation in the first place.
Marking a variable as volatile will just mean that you are forcing the value to be flushed to and from memory each time which in many cases just slows down your code as you've basically blown your cache performance.
c# and java AFAIK do redress this by making volatile adhere to 1) and 2) however the same cannot be said for c/c++ compilers so basically do with it as you see fit.
For some more in depth ( though not unbiased ) discussion on the subject read this
The comp.programming.threads FAQ has a classic explanation by Dave Butenhof:
Q56: Why don't I need to declare shared variables VOLATILE?
I'm concerned, however, about cases where both the compiler and the
threads library fulfill their respective specifications. A conforming
C compiler can globally allocate some shared (nonvolatile) variable to
a register that gets saved and restored as the CPU gets passed from
thread to thread. Each thread will have it's own private value for
this shared variable, which is not what we want from a shared
variable.
In some sense this is true, if the compiler knows enough about the
respective scopes of the variable and the pthread_cond_wait (or
pthread_mutex_lock) functions. In practice, most compilers will not try
to keep register copies of global data across a call to an external
function, because it's too hard to know whether the routine might
somehow have access to the address of the data.
So yes, it's true that a compiler that conforms strictly (but very
aggressively) to ANSI C might not work with multiple threads without
volatile. But someone had better fix it. Because any SYSTEM (that is,
pragmatically, a combination of kernel, libraries, and C compiler) that
does not provide the POSIX memory coherency guarantees does not CONFORM
to the POSIX standard. Period. The system CANNOT require you to use
volatile on shared variables for correct behavior, because POSIX
requires only that the POSIX synchronization functions are necessary.
So if your program breaks because you didn't use volatile, that's a BUG.
It may not be a bug in C, or a bug in the threads library, or a bug in
the kernel. But it's a SYSTEM bug, and one or more of those components
will have to work to fix it.
You don't want to use volatile, because, on any system where it makes
any difference, it will be vastly more expensive than a proper
nonvolatile variable. (ANSI C requires "sequence points" for volatile
variables at each expression, whereas POSIX requires them only at
synchronization operations -- a compute-intensive threaded application
will see substantially more memory activity using volatile, and, after
all, it's the memory activity that really slows you down.)
/---[ Dave Butenhof ]-----------------------[ butenhof#zko.dec.com ]---\
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Mr Butenhof covers much of the same ground in this usenet post:
The use of "volatile" is not sufficient to ensure proper memory
visibility or synchronization between threads. The use of a mutex is
sufficient, and, except by resorting to various non-portable machine
code alternatives, (or more subtle implications of the POSIX memory
rules that are much more difficult to apply generally, as explained in
my previous post), a mutex is NECESSARY.
Therefore, as Bryan explained, the use of volatile accomplishes
nothing but to prevent the compiler from making useful and desirable
optimizations, providing no help whatsoever in making code "thread
safe". You're welcome, of course, to declare anything you want as
"volatile" -- it's a legal ANSI C storage attribute, after all. Just
don't expect it to solve any thread synchronization problems for you.
All that's equally applicable to C++.
This is all that "volatile" is doing:
"Hey compiler, this variable could change AT ANY MOMENT (on any clock tick) even if there are NO LOCAL INSTRUCTIONS acting on it. Do NOT cache this value in a register."
That is IT. It tells the compiler that your value is, well, volatile- this value may be altered at any moment by external logic (another thread, another process, the Kernel, etc.). It exists more or less solely to suppress compiler optimizations that will silently cache a value in a register that it is inherently unsafe to EVER cache.
You may encounter articles like "Dr. Dobbs" that pitch volatile as some panacea for multi-threaded programming. His approach isn't totally devoid of merit, but it has the fundamental flaw of making an object's users responsible for its thread-safety, which tends to have the same issues as other violations of encapsulation.
According to my old C standard, “What constitutes an access to an object that has volatile- qualified type is implementation-defined”. So C compiler writers could have choosen to have "volatile" mean "thread safe access in a multi-process environment". But they didn't.
Instead, the operations required to make a critical section thread safe in a multi-core multi-process shared memory environment were added as new implementation-defined features. And, freed from the requirement that "volatile" would provide atomic access and access ordering in a multi-process environment, the compiler writers prioritised code-reduction over historical implemention-dependant "volatile" semantics.
This means that things like "volatile" semaphores around critical code sections, which do not work on new hardware with new compilers, might once have worked with old compilers on old hardware, and old examples are sometimes not wrong, just old.
I'm writing a C++ app.
I have a class variable that more than one thread is writing to.
In C++, anything that can be modified without the compiler "realizing" that it's being changed needs to be marked volatile right? So if my code is multi threaded, and one thread may write to a var while another reads from it, do I need to mark the var volaltile?
[I don't have a race condition since I'm relying on writes to ints being atomic]
Thanks!
C++ hasn't yet any provision for multithreading. In practice, volatile doesn't do what you mean (it has been designed for memory adressed hardware and while the two issues are similar they are different enough that volatile doesn't do the right thing -- note that volatile has been used in other language for usages in mt contexts).
So if you want to write an object in one thread and read it in another, you'll have to use synchronization features your implementation needs when it needs them. For the one I know of, volatile play no role in that.
FYI, the next standard will take MT into account, and volatile will play no role in that. So that won't change. You'll just have standard defined conditions in which synchronization is needed and standard defined way of achieving them.
Yes, volatile is the absolute minimum you'll need. It ensures that the code generator won't generate code that stores the variable in a register and always performs reads and writes from/to memory. Most code generators can provide atomicity guarantees on variables that have the same size as the native CPU word, they'll ensure the memory address is aligned so that the variable cannot straddle a cache-line boundary.
That is however not a very strong contract on modern multi-core CPUs. Volatile does not promise that another thread that runs on another core can see updates to the variable. That requires a memory barrier, usually an instruction that flushes the CPU cache. If you don't provide a barrier, the thread will in effect keep running until such a flush occurs naturally. That will eventually happen, the thread scheduler is bound to provide one. That can take milliseconds.
Once you've taken care of details like this, you'll eventually have re-invented a condition variable (aka event) that isn't likely to be any faster than the one provided by a threading library. Or as well tested. Don't invent your own, threading is hard enough to get right, you don't need the FUD of not being sure that the very basic primitives are solid.
volatile instruct the compiler not to optimize upon "intuition" of a variable value or usage since it could be optimize "from the outside".
volatile won't provide any synchronization however and your assumption of writes to int being atomic are all but realistic!
I'd guess we'd need to see some usage to know if volatile is needed in your case (or check the behavior of your program) or more importantly if you see some sort of synchronization.
I think that volatile only really applies to reading, especially reading memory-mapped I/O registers.
It can be used to tell the compiler to not assume that once it has read from a memory location that the value won't change:
while (*p)
{
// ...
}
In the above code, if *p is not written to within the loop, the compiler might decide to move the read outside the loop, more like this:
cached_p=*p
while (cached_p)
{
// ...
}
If p is a pointer to a memory-mapped I/O port, you would want the first version where the port is checked before the loop is entered every time.
If p is a pointer to memory in a multi-threaded app, you're still not guaranteed that writes are atomic.
Without locking you may still get 'impossible' re-orderings done by the compiler or processor. And there's no guarantee that writes to ints are atomic.
It would be better to use proper locking.
Volatile will solve your problem, ie. it will guarantee consistency among all the caches of the system. However it will be inefficiency since it will update the variable in memory for each R or W access. You might concider using a memory barrier, only whenever it is needed, instead.
If you are working with or gcc/icc have look on sync built-ins : http://gcc.gnu.org/onlinedocs/gcc-4.1.2/gcc/Atomic-Builtins.html
EDIT (mostly about pm100 comment):
I understand that my beliefs are not a reference so I found something to quote :)
The volatile keyword was devised to prevent compiler optimizations that might render code incorrect in the presence of certain asynchronous events. For example, if you declare a primitive variable as volatile, the compiler is not permitted to cache it in a register
From Dr Dobb's
More interesting :
Volatile fields are linearizable. Reading a volatile field is like acquiring a lock; the working memory is invalidated and the volatile field's current value is reread from memory. Writing a volatile field is like releasing a lock : the volatile field is immediately written back to memory.
(this is all about consistency, not about atomicity)
from The Art of multiprocessor programming, Maurice Herlihy & Nir Shavit
Lock contains memory synchronization code, if you don't lock, you must do something and using volatile keyword is probably the simplest thing you can do (even if it was designed for external devices with memory binded to the address space, it's not the point here)