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We believe that OSAtomicCompareAndSwap32() is correct. The test program is flawed.

The bug is that the compare and swap can succeed but the list head in oldListHead may have been recycled by someone else (that is, dequeued and enqueued again) so the list will become inconsistent -- newListHead will not refer to the correct new list head.

Erm, as I mentioned, the supposed bug is in OSDequeueAtomic, not in OSAtomicCompareAndSwap32...

You can not implement list queue and dequeue functions on ppc using a compare-and-swap function. You could do it if you could protect the entire operation inside a single lwarx/stwcx pair but the OSAtomic routines do not have a way to do that.

...and this is exactly the bug in OSDequeueAtomic that I was referring to, i.e. my "test_broken.cc" test program was created only to have an easily compilable and executable program that uses the algorithm from OSDequeueAtomic - and therefore crashes more or less immediately.

The reason that list_fixed.cc works is because it implements a completely different algorithm for dequeuing that avoids the race condition by temporarily unlinking the entire list.

It was a quick hack to see how one can layer atomic dequeue on atomic CAS and/or SWAP, however as somebody else already mentioned to me privately, my "fixed" version still crashes in some cases.

Further, your compare-and-swap routines are not correct because they do not take into account the various processor bugs (some of which are quite severe if not handled). I strongly advise people to

Thank you for this hint, I will hunt for documentation regarding these bugs as I might have other uses for an atomic unconditional swap, an operation that does not seem currently available. Regards, Colin _______________________________________________ Do not post admin requests to the list. They will be ignored. Darwin-kernel mailing list (Darwin-kernel@lists.apple.com) Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/darwin-kernel/site_archiver%40lists.a... This email sent to site_archiver@lists.apple.com