From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from bombadil.infradead.org (bombadil.infradead.org [IPv6:2001:1868:205::9]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id 3rFwL801RNzDq9J for ; Fri, 27 May 2016 02:57:20 +1000 (AEST) Date: Thu, 26 May 2016 18:57:12 +0200 From: Peter Zijlstra To: Pan Xinhui 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 Message-ID: <20160526165712.GT3205@twins.programming.kicks-ass.net> References: <1464164289-6124-1-git-send-email-xinhui.pan@linux.vnet.ibm.com> <1464164289-6124-6-git-send-email-xinhui.pan@linux.vnet.ibm.com> <20160526164729.GL3206@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii In-Reply-To: <20160526164729.GL3206@twins.programming.kicks-ass.net> List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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 > > Signed-off-by: Pan Xinhui > > --- > > 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. From mboxrd@z Thu Jan 1 00:00:00 1970 From: Peter Zijlstra 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 Message-ID: <20160526165712.GT3205@twins.programming.kicks-ass.net> References: <1464164289-6124-1-git-send-email-xinhui.pan@linux.vnet.ibm.com> <1464164289-6124-6-git-send-email-xinhui.pan@linux.vnet.ibm.com> <20160526164729.GL3206@twins.programming.kicks-ass.net> Mime-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Return-path: Content-Disposition: inline In-Reply-To: <20160526164729.GL3206@twins.programming.kicks-ass.net> List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Sender: virtualization-bounces@lists.linux-foundation.org Errors-To: virtualization-bounces@lists.linux-foundation.org To: Pan Xinhui Cc: jeremy@goop.org, mpe@ellerman.id.au, linux-kernel@vger.kernel.org, virtualization@lists.linux-foundation.org, chrisw@sous-sol.org, mingo@redhat.com, paulus@samba.org, benh@kernel.crashing.org, akataria@vmware.com, paulmck@linux.vnet.ibm.com, linuxppc-dev@lists.ozlabs.org List-Id: virtualization@lists.linuxfoundation.org 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 > > Signed-off-by: Pan Xinhui > > --- > > 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.