Question

I am protecting pointer with mutex for writes already like this

// thread1
if(pointer)
{
   boost::mutex::scoped_lock lock(pointer_mutex);
   if(pointer)
      pointer->DoStuff();
}

// thread2
if(pointer)
{
   boost::mutex::scoped_lock lock(pointer_mutex);
   if(pointer)
      pointer = anotherPointer;
}

// thread3
if(pointer)
{
   boost::mutex::scoped_lock lock(pointer_mutex);
   pointer = 0;
}

i do not want want to put that mutex outside of the block, because pointer is null 99.999 of the time.

this works fine with no crashes, but i not experienced enough declare it thread safe.

my question is:

Are if(pointer) pointer = 0; pointer = anotherPointer; atomic?

Thank you.

Answer 1

It’s not safe and neither is “double-checked locking”. Please do read this paper.