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[79.180.10.140]) by smtp.gmail.com with ESMTPSA id z132sm23730810wmb.21.2020.07.13.04.27.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 13 Jul 2020 04:27:05 -0700 (PDT) Date: Mon, 13 Jul 2020 07:27:01 -0400 From: "Michael S. Tsirkin" To: Will Deacon Cc: linux-kernel@vger.kernel.org, Joel Fernandes , Sami Tolvanen , Nick Desaulniers , Kees Cook , Marco Elver , "Paul E. McKenney" , Matt Turner , Ivan Kokshaysky , Richard Henderson , Peter Zijlstra , Alan Stern , Jason Wang , Arnd Bergmann , Boqun Feng , Catalin Marinas , Mark Rutland , linux-arm-kernel@lists.infradead.org, linux-alpha@vger.kernel.org, virtualization@lists.linux-foundation.org, kernel-team@android.com Subject: Re: [PATCH v3 07/19] vhost: Remove redundant use of read_barrier_depends() barrier Message-ID: <20200713072618-mutt-send-email-mst@kernel.org> References: <20200710165203.31284-1-will@kernel.org> <20200710165203.31284-8-will@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200710165203.31284-8-will@kernel.org> Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Jul 10, 2020 at 05:51:51PM +0100, Will Deacon wrote: > Since commit 76ebbe78f739 ("locking/barriers: Add implicit > smp_read_barrier_depends() to READ_ONCE()"), there is no need to use > smp_read_barrier_depends() outside of the Alpha architecture code. > > Unfortunately, there is precisely _one_ user in the vhost code, and > there isn't an obvious READ_ONCE() access making the barrier > redundant. However, on closer inspection (thanks, Jason), it appears > that vring synchronisation between the producer and consumer occurs via > the 'avail_idx' field, which is followed up by an rmb() in > vhost_get_vq_desc(), making the read_barrier_depends() redundant on > Alpha. > > Jason says: > > | I'm also confused about the barrier here, basically in driver side > | we did: > | > | 1) allocate pages > | 2) store pages in indirect->addr > | 3) smp_wmb() > | 4) increase the avail idx (somehow a tail pointer of vring) > | > | in vhost we did: > | > | 1) read avail idx > | 2) smp_rmb() > | 3) read indirect->addr > | 4) read from indirect->addr > | > | It looks to me even the data dependency barrier is not necessary > | since we have rmb() which is sufficient for us to the correct > | indirect->addr and driver are not expected to do any writing to > | indirect->addr after avail idx is increased > > Remove the redundant barrier invocation. > > Suggested-by: Jason Wang > Acked-by: Paul E. McKenney > Signed-off-by: Will Deacon I agree Acked-by: Michael S. Tsirkin Pls merge with the rest of the patchset. > --- > drivers/vhost/vhost.c | 5 ----- > 1 file changed, 5 deletions(-) > > diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c > index d7b8df3edffc..74d135ee7e26 100644 > --- a/drivers/vhost/vhost.c > +++ b/drivers/vhost/vhost.c > @@ -2092,11 +2092,6 @@ static int get_indirect(struct vhost_virtqueue *vq, > return ret; > } > iov_iter_init(&from, READ, vq->indirect, ret, len); > - > - /* We will use the result as an address to read from, so most > - * architectures only need a compiler barrier here. */ > - read_barrier_depends(); > - > count = len / sizeof desc; > /* Buffers are chained via a 16 bit next field, so > * we can have at most 2^16 of these. */ > -- > 2.27.0.383.g050319c2ae-goog