Is this a case for a mutex? - c++

I have a thread that captures data from a device. I start/stop the thread from a gui. At the moment, the thread periodically checks a bool member isCapturingEnabled in the appcontext. I toggle this bool member from the gui to stop the thread.
Is this a case where I should use a mutex? Because the capturing thread and the main thread might try to write and read the bool at the same time.

The problem you'll have is that without some sort of locking or memory barrier it is possible (for example) that the gui thread might set the bool to true but the thread won't actually see this due to compiler optimizations or optimizations at the CPU level.
What you need to do is write to the bool in such a way that the current state is loaded from memory, and the new state is correctly written back so that all threads will see the change. One way to do this is with a mutex, as you've identified. Others ways are to use memory barriers to ensure that you're accessing a correct view of memory. Most languages or OSs usually have some sort of API for manipulating memory up to the size of a word in an atomic fashion. For example, on Windows there's the InterlockedCompareExchange function.
However, in 95% of cases just wrapping the read/write in a mutex is good enough from a performance standpoint and is easy to reason about in terms of multithreaded correctness.

Related

C++ 17 shared_mutex : why read lock is even necessary for reading threads [duplicate]

I have a class that has a state (a simple enum) and that is accessed from two threads. For changing state I use a mutex (boost::mutex). Is it safe to check the state (e.g. compare state_ == ESTABLISHED) or do I have to use the mutex in this case too? In other words do I need the mutex when I just want to read a variable which could be concurrently written by another thread?
It depends.
The C++ language says nothing about threads or atomicity.
But on most modern CPU's, reading an integer is an atomic operation, which means that you will always read a consistent value, even without a mutex.
However, without a mutex, or some other form of synchronization, the compiler and CPU are free to reorder reads and writes, so anything more complex, anything involving accessing multiple variables, is still unsafe in the general case.
Assuming the writer thread updates some data, and then sets an integer flag to inform other threads that data is available, this could be reordered so the flag is set before updating the data. Unless you use a mutex or another form of memory barrier.
So if you want correct behavior, you don't need a mutex as such, and it's no problem if another thread writes to the variable while you're reading it. It'll be atomic unless you're working on a very unusual CPU. But you do need a memory barrier of some kind to prevent reordering in the compiler or CPU.
You have two threads, they exchange information, yes you need a mutex and you probably also need a conditional wait.
In your example (compare state_ == ESTABLISHED) indicates that thread #2 is waiting for thread #1 to initiate a connection/state. Without a mutex or conditionals/events, thread #2 has to poll the status continously.
Threads is used to increase performance (or improve responsiveness), polling usually results in decreased performance, either by consuming a lot of CPU or by introducing latencey due to the poll interval.
Yes. If thread a reads a variable while thread b is writing to it, you can read an undefined value. The read and write operation are not atomic, especially on a multi-processor system.
Generally speaking you don't, if your variable is declared with "volatile". And ONLY if it is a single variable - otherwise you should be really careful about possible races.
actually, there is no reason to lock access to the object for reading. you only want to lock it while writing to it. this is exactly what a reader-writer lock is. it doesn't lock the object as long as there are no write operations. it improves performance and prevents deadlocks. see the following links for more elaborate explanations :
wikipedia
codeproject
The access to the enum ( read or write) should be guarded.
Another thing:
If the thread contention is less and the threads belong to same process then Critical section would be better than mutex.

Do I need a mutex for reading?

I have a class that has a state (a simple enum) and that is accessed from two threads. For changing state I use a mutex (boost::mutex). Is it safe to check the state (e.g. compare state_ == ESTABLISHED) or do I have to use the mutex in this case too? In other words do I need the mutex when I just want to read a variable which could be concurrently written by another thread?
It depends.
The C++ language says nothing about threads or atomicity.
But on most modern CPU's, reading an integer is an atomic operation, which means that you will always read a consistent value, even without a mutex.
However, without a mutex, or some other form of synchronization, the compiler and CPU are free to reorder reads and writes, so anything more complex, anything involving accessing multiple variables, is still unsafe in the general case.
Assuming the writer thread updates some data, and then sets an integer flag to inform other threads that data is available, this could be reordered so the flag is set before updating the data. Unless you use a mutex or another form of memory barrier.
So if you want correct behavior, you don't need a mutex as such, and it's no problem if another thread writes to the variable while you're reading it. It'll be atomic unless you're working on a very unusual CPU. But you do need a memory barrier of some kind to prevent reordering in the compiler or CPU.
You have two threads, they exchange information, yes you need a mutex and you probably also need a conditional wait.
In your example (compare state_ == ESTABLISHED) indicates that thread #2 is waiting for thread #1 to initiate a connection/state. Without a mutex or conditionals/events, thread #2 has to poll the status continously.
Threads is used to increase performance (or improve responsiveness), polling usually results in decreased performance, either by consuming a lot of CPU or by introducing latencey due to the poll interval.
Yes. If thread a reads a variable while thread b is writing to it, you can read an undefined value. The read and write operation are not atomic, especially on a multi-processor system.
Generally speaking you don't, if your variable is declared with "volatile". And ONLY if it is a single variable - otherwise you should be really careful about possible races.
actually, there is no reason to lock access to the object for reading. you only want to lock it while writing to it. this is exactly what a reader-writer lock is. it doesn't lock the object as long as there are no write operations. it improves performance and prevents deadlocks. see the following links for more elaborate explanations :
wikipedia
codeproject
The access to the enum ( read or write) should be guarded.
Another thing:
If the thread contention is less and the threads belong to same process then Critical section would be better than mutex.

Overhead of pthread mutexes?

I'm trying to make a C++ API (for Linux and Solaris) thread-safe, so that its functions can be called from different threads without breaking internal data structures. In my current approach I'm using pthread mutexes to protect all accesses to member variables. This means that a simple getter function now locks and unlocks a mutex, and I'm worried about the overhead of this, especially as the API will mostly be used in single-threaded apps where any mutex locking seems like pure overhead.
So, I'd like to ask:
do you have any experience with performance of single-threaded apps that use locking versus those that don't?
how expensive are these lock/unlock calls, compared to eg. a simple "return this->isActive" access for a bool member variable?
do you know better ways to protect such variable accesses?
All modern thread implementations can handle an uncontended mutex lock entirely in user space (with just a couple of machine instructions) - only when there is contention, the library has to call into the kernel.
Another point to consider is that if an application doesn't explicitly link to the pthread library (because it's a single-threaded application), it will only get dummy pthread functions (which don't do any locking at all) - only if the application is multi-threaded (and links to the pthread library), the full pthread functions will be used.
And finally, as others have already pointed out, there is no point in protecting a getter method for something like isActive with a mutex - once the caller gets a chance to look at the return value, the value might already have been changed (as the mutex is only locked inside the getter method).
"A mutex requires an OS context switch. That is fairly expensive. "
This is not true on Linux, where mutexes are implemented using something called futex'es. Acquiring an uncontested (i.e., not already locked) mutex is, as cmeerw points out, a matter of a few simple instructions, and is typically in the area of 25 nanoseconds w/current hardware.
For more info:
Futex
Numbers everybody should know
This is a bit off-topic but you seem to be new to threading - for one thing, only lock where threads can overlap. Then, try to minimize those places. Also, instead of trying to lock every method, think of what the thread is doing (overall) with an object and make that a single call, and lock that. Try to get your locks as high up as possible (this again increases efficiency and may /help/ to avoid deadlocking). But locks don't 'compose', you have to mentally at least cross-organize your code by where the threads are and overlap.
I did a similar library and didn't have any trouble with lock performance. (I can't tell you exactly how they're implemented, so I can't say conclusively that it's not a big deal.)
I'd go for getting it right first (i.e. use locks) then worry about performance. I don't know of a better way; that's what mutexes were built for.
An alternative for single thread clients would be to use the preprocessor to build a non-locked vs locked version of your library. E.g.:
#ifdef BUILD_SINGLE_THREAD
inline void lock () {}
inline void unlock () {}
#else
inline void lock () { doSomethingReal(); }
inline void unlock () { doSomethingElseReal(); }
#endif
Of course, that adds an additional build to maintain, as you'd distribute both single and multithread versions.
I can tell you from Windows, that a mutex is a kernel object and as such incurs a (relatively) significant locking overhead. To get a better performing lock, when all you need is one that works in threads, is to use a critical section. This would not work across processes, just the threads in a single process.
However.. linux is quite a different beast to multi-process locking. I know that a mutex is implemented using the atomic CPU instructions and only apply to a process - so they would have the same performance as a win32 critical section - ie be very fast.
Of course, the fastest locking is not to have any at all, or to use them as little as possible (but if your lib is to be used in a heavily threaded environment, you will want to lock for as short a time as possible: lock, do something, unlock, do something else, then lock again is better than holding the lock across the whole task - the cost of locking isn't in the time taken to lock, but the time a thread sits around twiddling its thumbs waiting for another thread to release a lock it wants!)
A mutex requires an OS context switch. That is fairly expensive. The CPU can still do it hundreds of thousands of times per second without too much trouble, but it is a lot more expensive than not having the mutex there. Putting it on every variable access is probably overkill.
It also probably is not what you want. This kind of brute-force locking tends to lead to deadlocks.
do you know better ways to protect such variable accesses?
Design your application so that as little data as possible is shared. Some sections of code should be synchronized, probably with a mutex, but only those that are actually necessary. And typically not individual variable accesses, but tasks containing groups of variable accesses that must be performed atomically. (perhaps you need to set your is_active flag along with some other modifications. Does it make sense to set that flag and make no further changes to the object?)
I was curious about the expense of using a pthred_mutex_lock/unlock.
I had a scenario where I needed to either copy anywhere from 1500-65K bytes without using
a mutex or to use a mutex and do a single write of a pointer to the data needed.
I wrote a short loop to test each
gettimeofday(&starttime, NULL)
COPY DATA
gettimeofday(&endtime, NULL)
timersub(&endtime, &starttime, &timediff)
print out timediff data
or
ettimeofday(&starttime, NULL)
pthread_mutex_lock(&mutex);
gettimeofday(&endtime, NULL)
pthread_mutex_unlock(&mutex);
timersub(&endtime, &starttime, &timediff)
print out timediff data
If I was copying less than 4000 or so bytes, then the straight copy operation took less time. If however I was copying more than 4000 bytes, then it was less costly to do the mutex lock/unlock.
The timing on the mutex lock/unlock ran between 3 and 5 usec long including the time for
the gettimeofday for the currentTime which took about 2 usec
For member variable access, you should use read/write locks, which have slightly less overhead and allow multiple concurrent reads without blocking.
In many cases you can use atomic builtins, if your compiler provides them (if you are using gcc or icc __sync_fetch*() and the like), but they are notouriously hard to handle correctly.
If you can guarantee the access being atomic (for example on x86 an dword read or write is always atomic, if it is aligned, but not a read-modify-write), you can often avoid locks at all and use volatile instead, but this is non portable and requires knowledge of the hardware.
Well a suboptimal but simple approach is to place macros around your mutex locks and unlocks. Then have a compiler / makefile option to enable / disable threading.
Ex.
#ifdef THREAD_ENABLED
#define pthread_mutex_lock(x) ... //actual mutex call
#endif
#ifndef THREAD_ENABLED
#define pthread_mutex_lock(x) ... //do nothing
#endif
Then when compiling do a gcc -DTHREAD_ENABLED to enable threading.
Again I would NOT use this method in any large project. But only if you want something fairly simple.

Why do locks work?

If the locks make sure only one thread accesses the locked data at a time, then what controls access to the locking functions?
I thought that boost::mutex::scoped_lock should be at the beginning of each of my functions so the local variables don't get modified unexpectedly by another thread, is that correct? What if two threads are trying to acquire the lock at very close times? Won't the lock's local variables used internally be corrupted by the other thread?
My question is not boost-specific but I'll probably be using that unless you recommend another.
You're right, when implementing locks you need some way of guaranteeing that two processes don't get the lock at the same time. To do this, you need to use an atomic instruction - one that's guaranteed to complete without interruption. One such instruction is test-and-set, an operation that will get the state of a boolean variable, set it to true, and return the previously retrieved state.
What this does is this allows you to write code that continually tests to see if it can get the lock. Assume x is a shared variable between threads:
while(testandset(x));
// ...
// critical section
// this code can only be executed by once thread at a time
// ...
x = 0; // set x to 0, allow another process into critical section
Since the other threads continually test the lock until they're let into the critical section, this is a very inefficient way of guaranteeing mutual exclusion. However, using this simple concept, you can build more complicated control structures like semaphores that are much more efficient (because the processes aren't looping, they're sleeping)
You only need to have exclusive access to shared data. Unless they're static or on the heap, local variables inside functions will have different instances for different threads and there is no need to worry. But shared data (stuff accessed via pointers, for example) should be locked first.
As for how locks work, they're carefully designed to prevent race conditions and often have hardware level support to guarantee atomicity. IE, there are some machine language constructs guaranteed to be atomic. Semaphores (and mutexes) may be implemented via these.
The simplest explanation is that the locks, way down underneath, are based on a hardware instruction that is guaranteed to be atomic and can't clash between threads.
Ordinary local variables in a function are already specific to an individual thread. It's only statics, globals, or other data that can be simultaneously accessed by multiple threads that needs to have locks protecting it.
The mechanism that operates the lock controls access to it.
Any locking primitive needs to be able to communicate changes between processors, so it's usually implemented on top of bus operations, i.e., reading and writing to memory. It also needs to be structured such that two threads attempting to claim it won't corrupt its state. It's not easy, but you can usually trust that any OS implemented lock will not get corrupted by multiple threads.

In a multi-threaded C++ app, do I need a mutex to protect a simple boolean?

I have a multi-threaded C++ app which does 3D rendering with the OpenSceneGraph library. I'm planning to kick off OSG's render loop as a separate thread using boost::threads, passing a data structure containing shared state in to the thread. I'm trying to avoid anything too heavyweight (like mutexes) for synchronization, as the render loop needs to be pretty tight, and OSG itself tries to avoid having to ever lock. Most of the shared state is set before the thread is started, and never changed. I do have some data that does need to be changed, which I am planning to double-buffer. However, I have a simple boolean for signaling the thread to suspend rendering, and later resume rendering, and another to kill it. In both cases the app thread sets the bool, and the render thread only reads it. Do I need to synchronize access to these bools? As far as I can tell, the worse thing that could happen is the the render loop continues on for an extra frame before suspending or quitting.
In C++11 and later, which has standards-defined concurrency, use std::atomic<bool> for this purpose. From http://en.cppreference.com/w/cpp/atomic/atomic:
If one thread writes to an atomic object while another thread reads from it, the behavior is well-defined (see memory model for details on data races).
The following old answer may have been true at some time in the past with some compilers and some operating environments, but it should not be relied upon today:
You're right, in this case you won't need to synchronise the bools. You should declare them volatile though, to ensure that the compiler actually reads them from memory each time, instead of caching the previous read in a thread (that's a simplified explanation, but it should do for this purpose).
The following question has more information about this:
C++ Thread, shared data
Why not simply use an interlocked variable?
As for C++11 and later it is finally threads-aware and clearly states that modifying a bool (or other non-atomic variable) in one thread and accessing it at the same time in another one is undefined behavior.
In you case using std::atomic<bool> should be enough to make your program correct, saving you from using locks.
Do not use volatile. It has nothing to do with threads.
For more discussion look at Can I read a bool variable in a thread without mutex?
I don't think you need a fully fledged mutex here -- though the render thread will need to busy wait in the 'suspended' state if you aren't using a synchronization object that supports a wait primitive.
You should look into using the various interlocked exchange primitives though (InterlockedExchange under Windows). Not because read/writes from the bool are non-atomic, but to ensure that there are no weird behaviours the compiler reordering memory accesses on a single thread.
This thread has a little more info and discussion on thread-safety, especially for simple data types:
How can I create a thread-safe singleton pattern in Windows?