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 18:47:29 +0200 [thread overview]
Message-ID: <20160526164729.GL3206@twins.programming.kicks-ass.net> (raw)
In-Reply-To: <1464164289-6124-6-git-send-email-xinhui.pan@linux.vnet.ibm.com>
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.
next prev parent reply other threads:[~2016-05-26 16:47 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 [this message]
2016-05-26 16:57 ` Peter Zijlstra
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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