From 71c0b9a65cefa8c34eab83d337a1e3ec61fb7cc2 Mon Sep 17 00:00:00 2001 From: Will Deacon Date: Wed, 30 Oct 2019 16:22:17 +0000 Subject: [PATCH] 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. Acked-by: Michael S. Tsirkin Acked-by: Peter Zijlstra (Intel) Suggested-by: Jason Wang Acked-by: Paul E. McKenney Signed-off-by: Will Deacon --- drivers/vhost/vhost.c | 5 ----- 1 file changed, 5 deletions(-) diff --git a/drivers/vhost/vhost.c b/drivers/vhost/vhost.c index d7b8df3..74d135e 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. */ -- 2.7.4