Re: armv7-on-aarch64 stuck at urdlck

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).