[PATCH 06/18] vhost: Remove redundant use of read_barrier_depends() barrier

From: Will Deacon <will@kernel.org>
To: linux-kernel@vger.kernel.org
Cc: Mark Rutland <mark.rutland@arm.com>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Jason Wang <jasowang@redhat.com>,
	virtualization@lists.linux-foundation.org,
	Will Deacon <will@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Alan Stern <stern@rowland.harvard.edu>,
	Sami Tolvanen <samitolvanen@google.com>,
	Matt Turner <mattst88@gmail.com>,
	kernel-team@android.com, Marco Elver <elver@google.com>,
	Kees Cook <keescook@chromium.org>,
	"Paul E. McKenney" <paulmck@kernel.org>,
	Boqun Feng <boqun.feng@gmail.com>,
	Josh Triplett <josh@joshtriplett.org>,
	Ivan Kokshaysky <ink@jurassic.park.msu.ru>,
	linux-arm-kernel@lists.infradead.org,
	Richard Henderson <rth@twiddle.net>,
	Nick Desaulniers <ndesaulniers@google.com>,
	linux-alpha@vger.kernel.org
Subject: [PATCH 06/18] vhost: Remove redundant use of read_barrier_depends() barrier
Date: Tue, 30 Jun 2020 18:37:22 +0100	[thread overview]
Message-ID: <20200630173734.14057-7-will@kernel.org> (raw)
In-Reply-To: <20200630173734.14057-1-will@kernel.org>

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 <jasowang@redhat.com>
Acked-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Will Deacon <will@kernel.org>
---
 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. */
-- 
2.27.0.212.ge8ba1cc988-goog


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