Re: armv7-on-aarch64 stuck at urdlck
- Reply: Michal Meloun : "Re: armv7-on-aarch64 stuck at urdlck"
- In reply to: Michal Meloun : "Re: armv7-on-aarch64 stuck at urdlck"
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Date: Mon, 22 Jul 2024 22:50:31 UTC
On Mon, Jul 22, 2024 at 09:36:00PM +0200, Michal Meloun wrote: > IMHO, -O2 shouldn't be able to modify function arguments for public > functions, so <guessing> this memory corruption fits perfectly with the > observed behavior</guessing>. > > But , out of curiosity, a quick look at _thr_rwlock_tryrdlock() in > thr_umtx.h:208 makes me wonder: How is the "state" variable inside the loop > guaranteed to be updated? IMHO nothing inside the loop emits a global memory > modification attribute, so the compiler is free to move the assignment to a > "state" variable outside the loop. > I think that you are formally right, because there is only the _acq atomic in the loop body, an evil compiler is allowed to move all loads before the start of the loop iteration. But, since e.g. on arm32 atomic_cmpset_acq implementation contains dmb() which provides the full barrier both for compiler memory accesses, and for hw, it is not the case (for arm32).