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Date:   Tue, 30 Jun 2020 18:37:22 +0100
From:   Will Deacon <will@...nel.org>
To:     linux-kernel@...r.kernel.org
Cc:     Will Deacon <will@...nel.org>,
        Sami Tolvanen <samitolvanen@...gle.com>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Marco Elver <elver@...gle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Josh Triplett <josh@...htriplett.org>,
        Matt Turner <mattst88@...il.com>,
        Ivan Kokshaysky <ink@...assic.park.msu.ru>,
        Richard Henderson <rth@...ddle.net>,
        Peter Zijlstra <peterz@...radead.org>,
        Alan Stern <stern@...land.harvard.edu>,
        "Michael S. Tsirkin" <mst@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Arnd Bergmann <arnd@...db.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Catalin Marinas <catalin.marinas@....com>,
        Mark Rutland <mark.rutland@....com>,
        linux-arm-kernel@...ts.infradead.org, linux-alpha@...r.kernel.org,
        virtualization@...ts.linux-foundation.org, kernel-team@...roid.com
Subject: [PATCH 06/18] vhost: Remove redundant use of read_barrier_depends() barrier

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@...hat.com>
Acked-by: Paul E. McKenney <paulmck@...nel.org>
Signed-off-by: Will Deacon <will@...nel.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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