From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752183AbeAZNoS (ORCPT ); Fri, 26 Jan 2018 08:44:18 -0500 Received: from mx1.redhat.com ([209.132.183.28]:51288 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751412AbeAZNoQ (ORCPT ); Fri, 26 Jan 2018 08:44:16 -0500 Date: Fri, 26 Jan 2018 15:44:15 +0200 From: "Michael S. Tsirkin" To: Jason Wang Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org, John Fastabend , David Miller Subject: Re: [PATCH net-next 03/12] ptr_ring: READ/WRITE_ONCE for __ptr_ring_empty Message-ID: <20180126154257-mutt-send-email-mst@kernel.org> References: <1516923320-16959-1-git-send-email-mst@redhat.com> <1516923320-16959-4-git-send-email-mst@redhat.com> <2b7d17c0-ef6c-0aad-9b47-0fb8ad78cc3a@redhat.com> <20180126044231-mutt-send-email-mst@kernel.org> <51d01ba2-56f9-c68e-ec19-6799d2c87d21@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline Content-Transfer-Encoding: 8bit In-Reply-To: <51d01ba2-56f9-c68e-ec19-6799d2c87d21@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jan 26, 2018 at 11:19:58AM +0800, Jason Wang wrote: > > > On 2018年01月26日 10:44, Michael S. Tsirkin wrote: > > On Fri, Jan 26, 2018 at 10:37:58AM +0800, Jason Wang wrote: > > > > > > On 2018年01月26日 07:36, Michael S. Tsirkin wrote: > > > > Lockless __ptr_ring_empty requires that consumer head is read and > > > > written at once, atomically. Annotate accordingly to make sure compiler > > > > does it correctly. Switch locked callers to __ptr_ring_peek which does > > > > not support the lockless operation. > > > > > > > > Signed-off-by: Michael S. Tsirkin > > > > --- > > > > include/linux/ptr_ring.h | 11 ++++++++--- > > > > 1 file changed, 8 insertions(+), 3 deletions(-) > > > > > > > > diff --git a/include/linux/ptr_ring.h b/include/linux/ptr_ring.h > > > > index 8594c7b..9a72d8f 100644 > > > > --- a/include/linux/ptr_ring.h > > > > +++ b/include/linux/ptr_ring.h > > > > @@ -196,7 +196,9 @@ static inline void *__ptr_ring_peek(struct ptr_ring *r) > > > > */ > > > > static inline bool __ptr_ring_empty(struct ptr_ring *r) > > > > { > > > > - return !__ptr_ring_peek(r); > > > > + if (likely(r->size)) > > > > + return !r->queue[READ_ONCE(r->consumer_head)]; > > > > + return true; > > > > } > > > So after patch 8, __ptr_ring_peek() did: > > > > > > static inline void *__ptr_ring_peek(struct ptr_ring *r) > > > { > > >     if (likely(r->size)) > > >         return READ_ONCE(r->queue[r->consumer_head]); > > >     return NULL; > > > } > > > > > > Looks like a duplication. > > > > > > Thanks > > Nope - they are different. > > > > The reason is that __ptr_ring_peek does not need to read the consumer_head once > > since callers have a lock, > > I get this. > > > and __ptr_ring_empty does not need to read > > the queue once since it merely compares it to 0. > > > > Do this still work if it was called inside a loop? > > Thanks Sure because compiler does not know head didn't change. -- MST