In my multithreaded server I have somefunction(), which needs to protect two independent of each other global data using EnterCriticalSection.
somefunction()
{
EnterCriticalSection(&g_List);
...
EnterCriticalSection(&g_Variable);
...
LeaveCriticalSection(&g_Variable);
...
LeaveCriticalSection(&g_List);
}
Following the advice of better programmers i'm going to use a RAII wrapper. For example:
class Locker
{
public:
Locker(CSType& cs): m_cs(cs)
{
EnterCriticalSection(&m_cs);
}
~Locker()
{
LeaveCriticalSection(&m_cs);
}
private:
CSType& m_cs;
}
My question: Is it ok to transform somefunction() to this?
(putting 2 Locker in one function):
somefunction()
{
// g_List,g_Variable previously initialized via InitializeCriticalSection
Locker lock(g_List);
Locker lock(g_Variable);
...
...
}
?
Your current solution has potential dead lock case. If you have two (or more) CSTypes which will be locked in different order this way, you will end up in dead lock. Best way would be to lock them both atomically. You can see an example of this in boost thread library. shared_lock and unique_lock can be used in deferred mode so that first you prepare all raii objects for all mutex objects, and then lock them all atomically in one call to lock function.
As long as you keep lock order the same in your threads its OK. Do you really need to lock them both at the same time? Also with scoped lock you can add scopes to control when to unlock, something like this:
{
// use inner scopes to control lock duration
{
Locker lockList (g_list);
// do something
} // unlocked at the end
Locker lockVariable (g_variable);
// do something
}
Related
I have seen a lot of examples of code when developer uses std::unique_lock in new scope for automatically unlocking mutex:
...
// do some staff
{
std::unique_lock<std::mutex> lock(shared_resource_mutex);
// do some actions with shared resource
}
// do some staff
...
In my opinion it would be better to implement this behaviour using method unlock from std::unique_lock API in this way:
...
// do some actions
std::unique_lock<std::mutex> lock(shared_resource_mutex);
// do some actions with shared resource
lock.unlock();
// do some actions
...
Are these two fragments of code equivalent? For what purpose developers use the first variant? Maybe to emphasize (using parentheses) code that can not be executed parallel?
When the object is destroyed at the end of the scope, the lock is released. That's the whole point of RAII.
The good thing about using RAII is that you cannot forget to unlock and it doesn't matter how you leave the scope. If an exception is thrown for example, the lock will still be released.
If all you need is lock at construction and unlock at destruction, then std::scoped_lock is an even simpler/more appropriate class to use though.
I would say the former method is safer, more consistent and easier to read.
First consider safety:
void function()
{
std::unique_lock<std::shared_mutex> lock(mtx);
// exclusive lock stuff
lock.unlock();
// std::shared_lock<std::shared_mutex> lock(mtx); // whoops name in use
std::shared_lock<std::shared_mutex> lock2(mtx);
// read only shared lock stuff here
lock2.unlock(); // what if I forget to do this?
lock.lock(); // if I forgot to call lock2.unlock() undefined behavior
// back to the exclusive stuff
lock.unlock();
}
If you have different locks to acquire/release and you forget to call unlock() then you may end up trying to lock the same mutex twice from the same thread.
That is undefined behavior so it may go unnoticed but cause trouble.
And what if you call either lock() or unlock() on the wrong lock variable.... (say on lock2 rather than lock1?) the possibilities are frightening.
Consistency:
Also, not all lock types have a .unlock() function (std::scoped_lock, std::lock_guard) so it is good to be consistent with your coding style.
Easier to read:
It is also easier to see what code sections use locks which makes reasoning on the code simpler:
void function()
{
{
std::unique_lock<std::shared_mutex> lock(mtx);
// exclusive lock stuff
}
{
std::shared_lock<std::shared_mutex> lock(mtx);
// read only shared lock stuff here
}
{
std::unique_lock<std::shared_mutex> lock(mtx);
// back to the exclusive stuff
}
}
Both of your approaches are correct, and you might choose either of them depending on circumstance. For example, when using a condition_variable/lock combination it's often useful to be able to explicitly lock and unlock the lock.
Here's another approach that I find to be both expressive and safe:
#include <mutex>
template<class Mutex, class Function>
decltype(auto) with_lock(Mutex& m, Function&& f)
{
std::lock_guard<Mutex> lock(m);
return f();
}
std::mutex shared_resource_mutex;
void something()
{
with_lock(shared_resource_mutex, [&]
{
// some actions
});
// some other actions
}
I have a function foo() that acquires a critical section does some processing and releases the critical section.Now there are multiple control flows so in order to avoid remembering to release the lock i thought of wrapping it up in a class, so that the destructor would automatically free the lock.
class Lock
{
public:
LPCRITICAL_SECTION m_a;
Lock(CRITICAL_SECTION *a):m_a(a){EnterCriticalSection(a);}
~Lock(){LeaveCriticalSection(m_a);}
};
Now the problem is that i have control flows where i want to acquire the lock, do something and then free it, and then continue other processing.So i don't want to wait to free the lock till function ends when the destruction would kick in.Is there a way to achieve this.
Use a local block:
void myFunction() {
// do stuff
{
Lock l(&critsec);
// do stuff needing lock
}
// do more stuff
}
I've reached a point in my project that requires communication between threads on resources that very well may be written to, so synchronization is a must. However I don't really understand synchronization at anything other than the basic level.
Consider the last example in this link: http://www.bogotobogo.com/cplusplus/C11/7_C11_Thread_Sharing_Memory.php
#include <iostream>
#include <thread>
#include <list>
#include <algorithm>
#include <mutex>
using namespace std;
// a global variable
std::list<int>myList;
// a global instance of std::mutex to protect global variable
std::mutex myMutex;
void addToList(int max, int interval)
{
// the access to this function is mutually exclusive
std::lock_guard<std::mutex> guard(myMutex);
for (int i = 0; i < max; i++) {
if( (i % interval) == 0) myList.push_back(i);
}
}
void printList()
{
// the access to this function is mutually exclusive
std::lock_guard<std::mutex> guard(myMutex);
for (auto itr = myList.begin(), end_itr = myList.end(); itr != end_itr; ++itr ) {
cout << *itr << ",";
}
}
int main()
{
int max = 100;
std::thread t1(addToList, max, 1);
std::thread t2(addToList, max, 10);
std::thread t3(printList);
t1.join();
t2.join();
t3.join();
return 0;
}
The example demonstrates how three threads, two writers and one reader, accesses a common resource(list).
Two global functions are used: one which is used by the two writer threads, and one being used by the reader thread. Both functions use a lock_guard to lock down the same resource, the list.
Now here is what I just can't wrap my head around: The reader uses a lock in a different scope than the two writer threads, yet still locks down the same resource. How can this work? My limited understanding of mutexes lends itself well to the writer function, there you got two threads using the exact same function. I can understand that, a check is made right as you are about to enter the protected area, and if someone else is already inside, you wait.
But when the scope is different? This would indicate that there is some sort of mechanism more powerful than the process itself, some sort of runtime environment blocking execution of the "late" thread. But I thought there were no such things in c++. So I am at a loss.
What exactly goes on under the hood here?
Let’s have a look at the relevant line:
std::lock_guard<std::mutex> guard(myMutex);
Notice that the lock_guard references the global mutex myMutex. That is, the same mutex for all three threads. What lock_guard does is essentially this:
Upon construction, it locks myMutex and keeps a reference to it.
Upon destruction (i.e. when the guard's scope is left), it unlocks myMutex.
The mutex is always the same one, it has nothing to do with the scope. The point of lock_guard is just to make locking and unlocking the mutex easier for you. For example, if you manually lock/unlock, but your function throws an exception somewhere in the middle, it will never reach the unlock statement. So, doing it the manual way you have to make sure that the mutex is always unlocked. On the other hand, the lock_guard object gets destroyed automatically whenever the function is exited – regardless how it is exited.
myMutex is global, which is what is used to protect myList. guard(myMutex) simply engages the lock and the exit from the block causes its destruction, dis-engaging the lock. guard is just a convenient way to engage and dis-engage the lock.
With that out of the way, mutex does not protect any data. It just provides a way to protect data. It is the design pattern that protects data. So if I write my own function to modify the list as below, the mutex cannot protect it.
void addToListUnsafe(int max, int interval)
{
for (int i = 0; i < max; i++) {
if( (i % interval) == 0) myList.push_back(i);
}
}
The lock only works if all pieces of code that need to access the data engage the lock before accessing and disengage after they are done. This design-pattern of engaging and dis-engaging the lock before and after every access is what protects the data (myList in your case)
Now you would wonder, why use mutex at all, and why not, say, a bool. And yes you can, but you will have to make sure that the bool variable will exhibit certain characteristics including but not limited to the below list.
Not be cached (volatile) across multiple threads.
Read and write will be atomic operation.
Your lock can handle situation where there are multiple execution pipelines (logical cores, etc).
There are different synchronization mechanisms that provide "better locking" (across processes versus across threads, multiple processor versus, single processor, etc) at a cost of "slower performance", so you should always choose a locking mechanism which is just about enough for your situation.
Just to add onto what others here have said...
There is an idea in C++ called Resource Acquisition Is Initialization (RAII) which is this idea of binding resources to the lifetime of objects:
Resource Acquisition Is Initialization or RAII, is a C++ programming technique which binds the life cycle of a resource that must be acquired before use (allocated heap memory, thread of execution, open socket, open file, locked mutex, disk space, database connection—anything that exists in limited supply) to the lifetime of an object.
C++ RAII Info
The use of a std::lock_guard<std::mutex> class follows the RAII idea.
Why is this useful?
Consider a case where you don't use a std::lock_guard:
std::mutex m; // global mutex
void oops() {
m.lock();
doSomething();
m.unlock();
}
in this case, a global mutex is used and is locked before the call to doSomething(). Then once doSomething() is complete the mutex is unlocked.
One problem here is what happens if there is an exception? Now you run the risk of never reaching the m.unlock() line which releases the mutex to other threads.
So you need to cover the case where you run into an exception:
std::mutex m; // global mutex
void oops() {
try {
m.lock();
doSomething();
m.unlock();
} catch(...) {
m.unlock(); // now exception path is covered
// throw ...
}
}
This works but is ugly, verbose, and inconvenient.
Now lets write our own simple lock guard.
class lock_guard {
private:
std::mutex& m;
public:
lock_guard(std::mutex& m_):(m(m_)){ m.lock(); } // lock on construction
~lock_guard() { t.unlock(); }} // unlock on deconstruction
}
When the lock_guard object is destroyed, it will ensure that the mutex is unlocked.
Now we can use this lock_guard to handle the case from before in a better/cleaner way:
std::mutex m; // global mutex
void ok() {
lock_guard lk(m); // our simple lock guard, protects against exception case
doSomething();
} // when scope is exited our lock guard object is destroyed and the mutex unlocked
This is the same idea behind std::lock_guard.
Again this approach is used with many different types of resources which you can read more about by following the link on RAII.
This is precisely what a lock does. When a thread takes the lock, regardless of where in the code it does so, it must wait its turn if another thread holds the lock. When a thread releases a lock, regardless of where in the code it does so, another thread may acquire that lock.
Locks protect data, not code. They do it by ensuring all code that accesses the protected data does so while it holds the lock, excluding other threads from any code that might access that same data.
I have a situation where I'd like to do something like that shown below, but there doesn't seem to be a way of querying the mutex without changing its state. I don't want the someFunctionCalledRepeatedlyFromAnotherThread() to hang around waiting for the mutex to free up if it is locked. It must return immediately after performing some alternative action. I'm guessing this omission is there for safety, as the lock may free up in the time between querying it and the function returning. In my case, bad stuff will not happen if the lock is freed while doSomeAlternativeAction() is happening. The fact I'm in this situation probably means I'm doing something wrong, so how should I change my design?
class MyClass
{
std::mutex initMutex;
public:
void someInitializationFunction()
{
std::lock_guard<std::mutex> lock(initMutex);
// Do some work
}
void someFunctionCalledRepeatedlyFromAnotherThread()
{
if (initMutex.isLocked())
{
doSomeAlternativeAction();
return;
}
// otherwise . . .
std::lock_guard<std::mutex> lock(initMutex);
// carry on with work as usual
}
}
Asking mutex for its state is useless: it may be unlocked now, but by the time you get around to lock it, it may be locked. So it does not have such method.
It does, however, have a method try_lock() which locks it if it is not locked and returns true if it acquired the lock and false otherwise. And std::unique_lock (the more advanced version of std::lock_guard) has option to call it.
So you can do:
void someFunctionCalledRepeatedlyFromAnotherThread()
{
std::unique_lock<std::mutex> lock(initMutex, std::try_to_lock);
if(!lock.owns_lock())
{
doSomeAlternativeAction();
return;
}
// otherwise ... go ahead, you have the lock
}
Sounds like you want to use std::unique_lock instead of std::lock_guard. The try_lock method works similar to the TryEnterCriticalSection on Windows, whereby the function will atomically acquire the lock and return 'true' if it can, or just return 'false' if the lock cannot be acquired (and will not block). Refer to http://msdn.microsoft.com/en-us/library/hh921439.aspx and http://en.cppreference.com/w/cpp/thread/unique_lock/try_lock. Note that unique_lock also has other members available for trying to lock such as try_lock_for and try_lock_until.
Imagine you have a big function that locks/unlocks a mutex inside and you want to break the function into smaller functions:
#include <pthread.h>
class MyClass : public Uncopyable
{
public:
MyClass() : m_mutexBuffer(PTHREAD_MUTEX_INITIALIZER), m_vecBuffer() {}
~MyClass() {}
void MyBigFunction()
{
pthread_mutex_lock(&m_mutexBuffer);
if (m_vecBuffer.empty())
{
pthread_mutex_unlock(&m_mutexBuffer);
return;
}
// DoSomethingWithBuffer1();
unsigned char ucBcc = CalculateBcc(&m_vecBuffer[0], m_vecBuffer.size());
// DoSomethingWithBuffer2();
pthread_mutex_unlock(&m_mutexBuffer);
}
private:
void DoSomethingWithBuffer1()
{
// Use m_vecBuffer
}
void DoSomethingWithBuffer2()
{
// Use m_vecBuffer
}
private:
pthread_mutex_t m_mutexBuffer;
std::vector<unsigned char> m_vecBuffer;
};
How should I go about locking/unlocking the mutex inside the smaller functions?
Should I unlock the mutex first, then lock it straightaway and finally unlock it before returning?
void DoSomethingWithBuffer1()
{
pthread_mutex_unlock(&m_mutexBuffer);
pthread_mutex_lock(&m_mutexBuffer);
// Use m_vecBuffer
pthread_mutex_unlock(&m_mutexBuffer);
}
How should I go about locking/unlocking the mutex inside the smaller functions?
If your semantics require your mutex to be locked during the whole MyBigFunction() operation then you can't simply unlock it and relock it in the middle of the function.
My best bet would be to ignore the mutex in the smaller DoSomethingWithBuffer...() functions, and simply require that these functions are called with the mutex being already locked. This shouldn't be a problem since those functions are private.
On a side note, your mutex usage is incorrect: it is not exception safe, and you have code paths where you don't release the mutex. You should either use C++11's mutex and lock classes or boost's equivalents if you are using C++03. At worst if you can't use boost, write a small RAII wrapper to hold the lock.
In general, try to keep the regions of code within each lock to a minimum (to avoid contention), but avoid to unlock and immediatly re-lock the same mutex. Thus, if the smaller functions are not mutually exclusive, they should both use their own indepdenent mutices and only when they actually access the shared resource.
Another thing that should consider is to use RAII for locking and unlocking (as in C++11 with std::lock_guard<>), so that returning from a locked region (either directly or via an uncaught exception) does not leave you in a locked state.