io_uring: fix poll full SQ detection

io_uring_poll shouldn't signal EPOLLOUT | EPOLLWRNORM if the queue is
full; the old check would always signal EPOLLOUT | EPOLLWRNORM (unless
there were U32_MAX - 1 entries in the SQ queue).

Signed-off-by: Stefan Bühler <source@stbuehler.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -2576,7 +2576,8 @@ static __poll_t io_uring_poll(struct file *file, poll_table *wait)
 	poll_wait(file, &ctx->cq_wait, wait);
 	/* See comment at the top of this file */
 	smp_rmb();
-	if (READ_ONCE(ctx->sq_ring->r.tail) + 1 != ctx->cached_sq_head)
+	if (READ_ONCE(ctx->sq_ring->r.tail) - ctx->cached_sq_head !=
+	    ctx->sq_ring->ring_entries)
 		mask |= EPOLLOUT | EPOLLWRNORM;
 	if (READ_ONCE(ctx->cq_ring->r.head) != ctx->cached_cq_tail)
 		mask |= EPOLLIN | EPOLLRDNORM;