linux-kernel - Re: [PATCH 2/2] locking/pvqspinlock: Use try_cmpxchg() in qspinlock

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Message-ID: <CAFULd4ZhJonzsP1GexJOy1PTx4PCTZoU1ukQrpmaNusumU2FTA@mail.gmail.com>
Date: Thu, 11 Apr 2024 15:35:54 +0200
From: Uros Bizjak <ubizjak@...il.com>
To: Ingo Molnar <mingo@...nel.org>
Cc: linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>, 
	Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>, Waiman Long <longman@...hat.com>, 
	Boqun Feng <boqun.feng@...il.com>
Subject: Re: [PATCH 2/2] locking/pvqspinlock: Use try_cmpxchg() in qspinlock_paravirt.h

On Thu, Apr 11, 2024 at 3:24 PM Ingo Molnar <mingo@...nel.org> wrote:
>
>
> * Uros Bizjak <ubizjak@...il.com> wrote:
>
> > -     locked = cmpxchg_release(&lock->locked, _Q_LOCKED_VAL, 0);
> > -     if (likely(locked == _Q_LOCKED_VAL))
> > +     if (try_cmpxchg_release(&lock->locked, &locked, 0);
> >               return;                                   ^------------ ???
>
> This doesn't appear to be a particularly well-tested patch. ;-)

Ouch, embarrassing... oh it is a generic function, conditionally compiled with

#ifndef __pv_queued_spin_lock
#endif

and x86 defines its own function ...

I concentrated on different settings of _Q_PENDING_BITS and the above
slipped through.

Thanks,
Uros.