Interlocked and Memory Barriers

Question

I have a question about the following code sample (m_value isn\'t volatile, and every thread runs on a separate processor)

void Foo() // executed by thr         


        

Answer 1

Memory barriers don't particularly help you. They specify an ordering between memory operations, in this case each thread only has one memory operation so it doesn't matter. One typical scenario is writing non-atomically to fields in a structure, a memory barrier, then publishing the address of the structure to other threads. The Barrier guarantees that the writes to the structures members are seen by all CPUs before they get the address of it.

What you really need are atomic operations, ie. InterlockedXXX functions, or volatile variables in C#. If the read in Bar were atomic, you could guarantee that neither the compiler, nor the cpu, does any optimizations that prevent it from reading either the value before the write in Foo, or after the write in Foo depending on which gets executed first. Since you are saying that you "know" Foo's write happens before Bar's read, then Bar would always return true.

Without the read in Bar being atomic, it could be reading a partially updated value (ie. garbage), or a cached value (either from the compiler or from the CPU), both of which may prevent Bar from returning true which it should.

Most modern CPU's guarantee word aligned reads are atomic, so the real trick is that you have to tell the compiler that the read is atomic.