Acquire and release sound like bass fishing terms, but they also apply to memory models

Raymond Chen

Many of the normal interlocked operations come with variants called InterlockedXxxAcquire and InterlockedXxxRelease. What do the terms Acquire and Release mean here?

They have to do with the memory model and how aggressively the CPU can reorder operations around it.

An operation with acquire semantics is one which does not permit subsequent memory operations to be advanced before it. Conversely, an operation with release semantics is one which does not permit preceding memory operations to be delayed past it. (This is pretty much the same thing that MSDN says on the subject of Acquire and Release Semantics.)

Consider the following code fragment:

int adjustment = CalculateAdjustment();
while (InterlockedCompareExchangeAcquire(&lock, 1, 0) != 0)
  { /* spin lock */ }
for (Node *node = ListHead; node; node = node->Next)
   node->value += adjustment;
InterlockedExchangeRelease(&lock, 0);

Applying Acquire semantics to the first operation operation ensures that the operations on the linked list are performed only after the lock variable has been updated. This is obviously desired here, since the purpose of the updating the lock variable is ensure that no other threads are updating the list while we’re walking it. Only after we have successfully set the lock to 1 is it safe to read from ListHead. On the other hand, the Acquire operation imposes no constraints upon when the store to the adjustment variable can be completed to memory. (Of course, there may very well be other constraints on the adjustment variable, but the Acquire does not add any new constraints.)

Conversely, Release semantics for an interlocked operation prevent pending memory operations from being delayed past the operation. In our example, this means that the stores to node->value must all complete before the interlocked variable’s value changes back to zero. This is also desired, because the purpose of the lock is to control access to the linked list. If we had completed the stores after the lock was released, then somebody else could have snuck in, taken the lock, and, say, deleted an entry from the linked list. And then when our pending writes completed, they would end up writing to memory that has been freed. Oops.

The easy way to remember the difference between Acquire and Release is that Acquire is typically used when you are acquiring a resource (in this case, taking a lock), whereas Release is typically used when you are releasing the resource.

As the MSDN article on acquire and release semantics already notes, the plain versions of the interlocked functions impose both acquire and release semantics.

Bonus reading: Kang Su discusses how VC2005 converts volatile memory accesses into acquires and releases.

[Raymond is currently away; this message was pre-recorded.]

Raymond Chen

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