From: Peter Zijlstra <peterz@infradead•org>
To: Pan Xinhui <xinhui.pan@linux•vnet.ibm.com>
Cc: linux-kernel@vger•kernel.org, linuxppc-dev@lists•ozlabs.org,
virtualization@lists•linux-foundation.org,
benh@kernel•crashing.org, paulus@samba•org, mpe@ellerman•id.au,
mingo@redhat•com, paulmck@linux•vnet.ibm.com, jeremy@goop•org,
chrisw@sous-sol•org, akataria@vmware•com, rusty@rustcorp•com.au
Subject: Re: [PATCH v3 5/6] pv-qspinlock: use cmpxchg_release in __pv_queued_spin_unlock
Date: Thu, 26 May 2016 18:57:12 +0200 [thread overview]
Message-ID: <20160526165712.GT3205@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <20160526164729.GL3206@twins.programming.kicks-ass.net>
On Thu, May 26, 2016 at 06:47:29PM +0200, Peter Zijlstra wrote:
> On Wed, May 25, 2016 at 04:18:08PM +0800, Pan Xinhui wrote:
> > cmpxchg_release is light-wight than cmpxchg, we can gain a better
> > performace then. On some arch like ppc, barrier impact the performace
> > too much.
> >
> > Suggested-by: Boqun Feng <boqun.feng@gmail•com>
> > Signed-off-by: Pan Xinhui <xinhui.pan@linux•vnet.ibm.com>
> > ---
> > kernel/locking/qspinlock_paravirt.h | 2 +-
> > 1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/kernel/locking/qspinlock_paravirt.h b/kernel/locking/qspinlock_paravirt.h
> > index a5b1248..2bbffe4 100644
> > --- a/kernel/locking/qspinlock_paravirt.h
> > +++ b/kernel/locking/qspinlock_paravirt.h
> > @@ -614,7 +614,7 @@ __visible void __pv_queued_spin_unlock(struct qspinlock *lock)
> > * unhash. Otherwise it would be possible to have multiple @lock
> > * entries, which would be BAD.
> > */
> > - locked = cmpxchg(&l->locked, _Q_LOCKED_VAL, 0);
> > + locked = cmpxchg_release(&l->locked, _Q_LOCKED_VAL, 0);
> > if (likely(locked == _Q_LOCKED_VAL))
> > return;
>
> This patch fails to explain _why_ it can be relaxed.
>
> And seeing how this cmpxchg() can actually unlock the lock, I don't see
> how this can possibly be correct. Maybe cmpxchg_release(), but relaxed
> seems very wrong.
Clearly I need to stop working for the day, I cannea read. You're doing
release, not relaxed.
Still Changelog needs improvement.
next prev parent reply other threads:[~2016-05-26 16:57 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-05-25 8:18 [PATCH v3 0/6] powerpc use pv-qpsinlock as the default spinlock implemention Pan Xinhui
2016-05-25 8:18 ` [PATCH v3 1/6] qspinlock: powerpc support qspinlock Pan Xinhui
2016-05-25 8:18 ` [PATCH v3 2/6] powerpc: pseries/Kconfig: Add qspinlock build config Pan Xinhui
2016-05-25 8:18 ` [PATCH v3 3/6] powerpc: lib/locks.c: Add cpu yield/wake helper function Pan Xinhui
2016-05-25 8:18 ` [PATCH v3 4/6] pv-qspinlock: powerpc support pv-qspinlock Pan Xinhui
2016-05-25 8:18 ` [PATCH v3 5/6] pv-qspinlock: use cmpxchg_release in __pv_queued_spin_unlock Pan Xinhui
2016-05-26 16:47 ` Peter Zijlstra
2016-05-26 16:57 ` Peter Zijlstra [this message]
2016-05-27 10:34 ` xinhui
2016-05-25 8:18 ` [PATCH v3 6/6] powerpc: pseries: Add pv-qspinlock build config/make Pan Xinhui
2016-05-26 16:50 ` [PATCH v3 0/6] powerpc use pv-qpsinlock as the default spinlock implemention Peter Zijlstra
2016-05-27 10:52 ` xinhui
2016-05-26 16:54 ` Peter Zijlstra
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