io_uring: fix race condition when sq threads goes sleeping

commit 0d7bae69c574c5f25802f8a71252e7d66933a3ab upstream.

Reading the SQ tail needs to come after setting IORING_SQ_NEED_WAKEUP in
flags; there is no cheap barrier for ordering a store before a load, a
full memory barrier is required.

Userspace needs a full memory barrier between updating SQ tail and
checking for the IORING_SQ_NEED_WAKEUP too.
Signed-off-by: NStefan Bühler <source@stbuehler.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>
Signed-off-by: NJoseph Qi <joseph.qi@linux.alibaba.com>
Reviewed-by: NJeffle Xu <jefflexu@linux.alibaba.com>
Acked-by: NCaspar Zhang <caspar@linux.alibaba.com>

fs/io_uring.c

@@ -1865,7 +1865,8 @@ static int io_sq_thread(void *data)
 
 			/* Tell userspace we may need a wakeup call */
 			ctx->sq_ring->flags |= IORING_SQ_NEED_WAKEUP;
-			smp_wmb();
+			/* make sure to read SQ tail after writing flags */
+			smp_mb();
 
 			if (!io_get_sqring(ctx, &sqes[0])) {
 				if (kthread_should_stop()) {