io_uring: further deduplicate #CQ events calc

Apparently, there is one more place hand coded calculation of number of
CQ events in the ring. Use __io_cqring_events() helper in
io_get_cqring() as well. Naturally, assembly stays identical.
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1700,21 +1700,25 @@ static inline bool io_sqring_full(struct io_ring_ctx *ctx)
 	return READ_ONCE(r->sq.tail) - ctx->cached_sq_head == r->sq_ring_entries;
 }
 
+static inline unsigned int __io_cqring_events(struct io_ring_ctx *ctx)
+{
+	return ctx->cached_cq_tail - READ_ONCE(ctx->rings->cq.head);
+}
+
 static struct io_uring_cqe *io_get_cqring(struct io_ring_ctx *ctx)
 {
 	struct io_rings *rings = ctx->rings;
 	unsigned tail;
 
-	tail = ctx->cached_cq_tail;
 	/*
 	 * 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(rings->cq.head) == rings->cq_ring_entries)
+	if (__io_cqring_events(ctx) == rings->cq_ring_entries)
 		return NULL;
 
-	ctx->cached_cq_tail++;
+	tail = ctx->cached_cq_tail++;
 	return &rings->cqes[tail & ctx->cq_mask];
 }
 
@@ -1729,11 +1733,6 @@ static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx)
 	return io_wq_current_is_worker();
 }
 
-static inline unsigned __io_cqring_events(struct io_ring_ctx *ctx)
-{
-	return ctx->cached_cq_tail - READ_ONCE(ctx->rings->cq.head);
-}
-
 static void io_cqring_ev_posted(struct io_ring_ctx *ctx)
 {
 	/* see waitqueue_active() comment */