From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Michael S. Tsirkin" Subject: Re: [PATCH net-next 03/12] ptr_ring: READ/WRITE_ONCE for __ptr_ring_empty Date: Fri, 26 Jan 2018 04:44:12 +0200 Message-ID: <20180126044231-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> Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 8bit Cc: linux-kernel@vger.kernel.org, netdev@vger.kernel.org, John Fastabend , David Miller To: Jason Wang Return-path: Content-Disposition: inline In-Reply-To: <2b7d17c0-ef6c-0aad-9b47-0fb8ad78cc3a@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org 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, and __ptr_ring_empty does not need to read the queue once since it merely compares it to 0. -- MST