io_uring: only call io_cqring_ev_posted() if events were posted

This normally doesn't cause any extra harm, but it does mean that we'll
increment the eventfd notification count, if one has been registered
with the ring. This can confuse applications, when they see more
notifications on the eventfd side than are available in the ring.

Do the nice thing and only increment this count, if we actually posted
(or even overflowed) events.
Reported-and-tested-by: Dan Melnic <dmm@fb.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1779,12 +1779,13 @@ static bool __io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force,
 	struct io_kiocb *req, *tmp;
 	struct io_uring_cqe *cqe;
 	unsigned long flags;
-	bool all_flushed;
+	bool all_flushed, posted;
 	LIST_HEAD(list);
 
 	if (!force && __io_cqring_events(ctx) == rings->cq_ring_entries)
 		return false;
 
+	posted = false;
 	spin_lock_irqsave(&ctx->completion_lock, flags);
 	list_for_each_entry_safe(req, tmp, &ctx->cq_overflow_list, compl.list) {
 		if (!io_match_task(req, tsk, files))
@@ -1804,6 +1805,7 @@ static bool __io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force,
 			WRITE_ONCE(ctx->rings->cq_overflow,
 				   ctx->cached_cq_overflow);
 		}
+		posted = true;
 	}
 
 	all_flushed = list_empty(&ctx->cq_overflow_list);
@@ -1813,9 +1815,11 @@ static bool __io_cqring_overflow_flush(struct io_ring_ctx *ctx, bool force,
 		ctx->rings->sq_flags &= ~IORING_SQ_CQ_OVERFLOW;
 	}
 
-	io_commit_cqring(ctx);
+	if (posted)
+		io_commit_cqring(ctx);
 	spin_unlock_irqrestore(&ctx->completion_lock, flags);
-	io_cqring_ev_posted(ctx);
+	if (posted)
+		io_cqring_ev_posted(ctx);
 
 	while (!list_empty(&list)) {
 		req = list_first_entry(&list, struct io_kiocb, compl.list);