c++ - does presence of mutex help getting rid of volatile key word ?

Question

I have a multi-R/W lock class that keeps the read, write and pending read , pending write counters. A mutex guards them from multiple threads.

My question is Do we still need the counters to be declared as volatile so that the compiler won't screw it up while doing the optimization.

Or does the compiler takes into account that the counters are guarded by the mutex.

I understand that the mutex is a run time mechanism to for synchronization and "volatile" keyword is a compile time indication to the compiler to do the right thing while doing the optimizations.

Regards, -Jay.

Answer 1

There are 2 basically unrelated items here, that are always confused.

• volatile
• threads, locks, memory barriers, etc.

volatile is used to tell the compiler to produce code to read the variable from memory, not from a register. And to not reorder the code around. In general, not to optimize or take 'short-cuts'.

memory barriers (supplied by mutexes, locks, etc), as quoted from Herb Sutter in another answer, are for preventing the CPU from reordering read/write memory requests, regardless of how the compiler said to do it. ie don't optimize, don't take short cuts - at the CPU level.

Similar, but in fact very different things.

In your case, and in most cases of locking, the reason that volatile is NOT necessary, is because of function calls being made for the sake of locking. ie:

Normal function calls affecting optimizations:

external void library_func(); // from some external library

global int x;

int f()
{
   x = 2;
   library_func();
   return x; // x is reloaded because it may have changed
}

unless the compiler can examine library_func() and determine that it doesn't touch x, it will re-read x on the return. This is even WITHOUT volatile.

Threading:

int f(SomeObject & obj)
{
   int temp1;
   int temp2;
   int temp3;

   int temp1 = obj.x;

   lock(obj.mutex); // really should use RAII
      temp2 = obj.x;
      temp3 = obj.x;
   unlock(obj.mutex);

   return temp;
}

After reading obj.x for temp1, the compiler is going to re-read obj.x for temp2 - NOT because of the magic of locks - but because it is unsure whether lock() modified obj. You could probably set compiler flags to aggressively optimize (no-alias, etc) and thus not re-read x, but then a bunch of your code would probably start failing.

For temp3, the compiler (hopefully) won't re-read obj.x. If for some reason obj.x could change between temp2 and temp3, then you would use volatile (and your locking would be broken/useless).

Lastly, if your lock()/unlock() functions were somehow inlined, maybe the compiler could evaluate the code and see that obj.x doesn't get changed. But I guarantee one of two things here: - the inline code eventually calls some OS level lock function (thus preventing evaluation) or - you call some asm memory barrier instructions (ie that are wrapped in inline functions like __InterlockedCompareExchange) that your compiler will recognize and thus avoid reordering.

EDIT: P.S. I forgot to mention - for pthreads stuff, some compilers are marked as "POSIX compliant" which means, among other things, that they will recognize the pthread_ functions and not do bad optimizations around them. ie even though the C++ standard doesn't mention threads yet, those compilers do (at least minimally).

So, short answer

you don't need volatile.