When to use volatile with multi threading?

Question

If there are two threads accessing a global variable then many tutorials say make the variable volatile to prevent the compiler caching the variable in a register and it thus not getting updated correctly. However two threads both accessing a shared variable is something which calls for protection via a mutex isn't it? But in that case, between the thread locking and releasing the mutex the code is in a critical section where only that one thread can access the variable, in which case the variable doesn't need to be volatile?

So therefore what is the use/purpose of volatile in a multi-threaded program?

Answer 1

You need volatile and possibly locking.

volatile tells the optimiser that the value can change asynchronously, thus

volatile bool flag = false;

while (!flag) {
    /*do something*/
}

will read flag every time around the loop.

If you turn optimisation off or make every variable volatile a program will behave the same but slower. volatile just means 'I know you may have just read it and know what it says, but if I say read it then read it.

Locking is a part of the program. So ,by the way, if you are implementing semaphores then among other things they must be volatile. (Don't try it, it is hard, will probably need a little assembler or the new atomic stuff, and it has already been done.)

Answer 2

Short & quick answer: volatile is (nearly) useless for platform-agnostic, multithreaded application programming. It does not provide any synchronization, it does not create memory fences, nor does it ensure the order of execution of operations. It does not make operations atomic. It does not make your code magically thread safe. volatile may be the single-most misunderstood facility in all of C++. See this, this and this for more information about volatile

On the other hand, volatile does have some use that may not be so obvious. It can be used much in the same way one would use const to help the compiler show you where you might be making a mistake in accessing some shared resource in a non-protected way. This use is discussed by Alexandrescu in this article. However, this is basically using the C++ type system in a way that is often viewed as a contrivance and can evoke Undefined Behavior.

volatile was specifically intended to be used when interfacing with memory-mapped hardware, signal handlers and the setjmp machine code instruction. This makes volatile directly applicable to systems-level programming rather than normal applications-level programming.

The 2003 C++ Standard does not say that volatile applies any kind of Acquire or Release semantics on variables. In fact, the Standard is completely silent on all matters of multithreading. However, specific platforms do apply Acquire and Release semantics on volatile variables.

[Update for C++11]

The C++11 Standard now does acknowledge multithreading directly in the memory model and the lanuage, and it provides library facilities to deal with it in a platform-independant way. However the semantics of volatile still have not changed. volatile is still not a synchronization mechanism. Bjarne Stroustrup says as much in TCPPPL4E:

Do not use volatile except in low-level code that deals directly with hardware.

Do not assume volatile has special meaning in the memory model. It does not. It is not -- as in some later languages -- a synchronization mechanism. To get synchronization, use atomic, a mutex, or a condition_variable.

[/End update]

The above all applies the the C++ language itself, as defined by the 2003 Standard (and now the 2011 Standard). Some specific platforms however do add additional functionality or restrictions to what volatile does. For example, in MSVC 2010 (at least) Acquire and Release semantics do apply to certain operations on volatile variables. From the MSDN:

When optimizing, the compiler must maintain ordering among references to volatile objects as well as references to other global objects. In particular,

A write to a volatile object (volatile write) has Release semantics; a reference to a global or static object that occurs before a write to a volatile object in the instruction sequence will occur before that volatile write in the compiled binary.

A read of a volatile object (volatile read) has Acquire semantics; a reference to a global or static object that occurs after a read of volatile memory in the instruction sequence will occur after that volatile read in the compiled binary.

However, you might take note of the fact that if you follow the above link, there is some debate in the comments as to whether or not acquire/release semantics actually apply in this case.

Answer 3

Volatile is occasionally useful for the following reason: this code:

/* global */ bool flag = false;

while (!flag) {}

is optimized by gcc to:

if (!flag) { while (true) {} }

Which is obviously incorrect if the flag is written to by the other thread. Note that without this optimization the synchronization mechanism probably works (depending on the other code some memory barriers may be needed) - there is no need for a mutex in 1 producer - 1 consumer scenario.

Otherwise the volatile keyword is too weird to be useable - it does not provide any memory ordering guarantees wrt both volatile and non-volatile accesses and does not provide any atomic operations - i.e. you get no help from the compiler with volatile keyword except disabled register caching.

Answer 4

#include <iostream>
#include <thread>
#include <unistd.h>
using namespace std;

bool checkValue = false;

int main()
{
    std::thread writer([&](){
            sleep(2);
            checkValue = true;
            std::cout << "Value of checkValue set to " << checkValue << std::endl;
        });

    std::thread reader([&](){
            while(!checkValue);
        });

    writer.join();
    reader.join();
}

Once an interviewer who also believed that volatile is useless argued with me that Optimisation wouldn't cause any issues and was referring to different cores having separate cache lines and all that (didn't really understand what he was exactly referring to). But this piece of code when compiled with -O3 on g++ (g++ -O3 thread.cpp -lpthread), it shows undefined behaviour. Basically if the value gets set before the while check it works fine and if not it goes into a loop without bothering to fetch the value (which was actually changed by the other thread). Basically i believe the value of checkValue only gets fetched once into the register and never gets checked again under the highest level of optimisation. If its set to true before the fetch, it works fine and if not it goes into a loop. Please correct me if am wrong.