Atomic Instructions and Variable Update visibility
- by dsimcha
On most common platforms (the most important being x86; I understand that some platforms have extremely difficult memory models that provide almost no guarantees useful for multithreading, but I don't care about rare counter-examples), is the following code safe?
Thread 1:
someVariable = doStuff();
atomicSet(stuffDoneFlag, 1);
Thread 2:
while(!atomicRead(stuffDoneFlag)) {} // Wait for stuffDoneFlag to be set.
doMoreStuff(someVariable);
Assuming standard, reasonable implementations of atomic ops:
Is Thread 1's assignment to someVariable guaranteed to complete before atomicSet() is called?
Is Thread 2 guaranteed to see the assignment to someVariable before calling doMoreStuff() provided it reads stuffDoneFlag atomically?
Edits:
The implementation of atomic ops I'm using contains the x86 LOCK instruction in each
operation, if that helps.
Assume stuffDoneFlag is properly cleared somehow. How isn't important.
This is a very simplified example. I created it this way so that you wouldn't have to understand the whole context of the problem to answer it. I know it's not efficient.
