io_uring: remove unnecessary barrier before reading cq head

mainline inclusion
from mainline-5.1
commit 115e12e5
category: feature
bugzilla: https://bugzilla.openeuler.org/show_bug.cgi?id=27
CVE: NA
---------------------------

The memory operations before reading cq head are unrelated and we
don't care about their order.

Document that the control dependency in combination with READ_ONCE and
WRITE_ONCE forms a barrier we need.
Signed-off-by: NStefan Bühler <source@stbuehler.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NZhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Nyangerkun <yangerkun@huawei.com>
Reviewed-by: Nzhangyi (F) <yi.zhang@huawei.com>
Signed-off-by: NCheng Jian <cj.chengjian@huawei.com>

fs/io_uring.c

@@ -431,8 +431,11 @@ static struct io_uring_cqe *io_get_cqring(struct io_ring_ctx *ctx)
 	unsigned tail;
 
 	tail = ctx->cached_cq_tail;
-	/* See comment at the top of the file */
-	smp_rmb();
+	/*
+	 * writes to the cq entry need to come after reading head; the
+	 * control dependency is enough as we're using WRITE_ONCE to
+	 * fill the cq entry
+	 */
 	if (tail - READ_ONCE(ring->r.head) == ring->ring_entries)
 		return NULL;