Re: PR/57437 CVS commit: src/lib/libpthread

[Taylor R Campbell <riastradh%NetBSD.org@localhost>]

> Date: Fri, 26 May 2023 08:03:18 +0000
> From: Vasily Dybala <Vasily.Dybala%kaspersky.com@localhost>
> 
> Fix looks ok, but pthread__smt_wake() call from
> pthread_mutex_unlock() will call SEV instruction which does not have
> paired WFE in pthread_mutex_lock().

We could add a call to pthread__smt_wake in pthread_mutex_unlock to
match a pthread__smt_wait in pthread__mutex_pause, but that's not
enough on its own, because currently pthread__mutex_spin calls
pthread__mutex_pause in a loop and might just WFE again without
realizing it can make progress after it has been woken by SEV.

So we would need to restructure pthread__mutex_spin so that it calls
pthread__smt_wait exactly once after it examines the owner and
determines the owner is still running on a CPU.

That may be worthwhile, but it's a little more involved to implement,
because the surrounding logic has to be conditionalized on which style
of spin loop.  So I picked the less-risky and less-involved option to
commit now.  We can try the WFE/SEV approach for pthread_mutex (and
rwlock and, if it's still relevant, simple lock) later.

> I will check patch during next week.

Great, thanks!