From 2c5d763c1939fbd130452ee0d4d1a44b5dd97bb7 Mon Sep 17 00:00:00 2001
From: Jens Axboe <axboe@kernel.dk>
Date: Sat, 21 Aug 2021 07:21:19 -0600
Subject: [PATCH] io_uring: add clarifying comment for io_cqring_ev_posted()
Git-commit: 2c5d763c1939fbd130452ee0d4d1a44b5dd97bb7
Patch-mainline: v5.15-rc1
References: bsc#1205205

We've previously had an issue where overflow flush unconditionally calls
io_cqring_ev_posted() even if it didn't flush any events to the ring,
causing wake and eventfd increment where no new events are available.
Some applications don't like that, see commit b18032bb0a88 for details.

This came up in discussion for another patch recently, hence add a
comment detailing what the relationship between calling the events
posted helper and CQ ring entries is.

Link: https://lore.kernel.org/io-uring/77a44fce-c831-16a6-8e80-9aee77f496a2@kernel.dk/
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Signed-off-by: Gabriel Krisman Bertazi <krisman@suse.de>
---
 fs/io_uring.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/fs/io_uring.c b/fs/io_uring.c
index c6f1afa5ec04..8d263209b508 100644
--- a/fs/io_uring.c
+++ b/fs/io_uring.c
@@ -1526,6 +1526,13 @@ static inline bool io_should_trigger_evfd(struct io_ring_ctx *ctx)
 	return !ctx->eventfd_async || io_wq_current_is_worker();
 }
 
+/*
+ * This should only get called when at least one event has been posted.
+ * Some applications rely on the eventfd notification count only changing
+ * IFF a new CQE has been added to the CQ ring. There's no depedency on
+ * 1:1 relationship between how many times this function is called (and
+ * hence the eventfd count) and number of CQEs posted to the CQ ring.
+ */
 static void io_cqring_ev_posted(struct io_ring_ctx *ctx)
 {
 	/*
-- 
2.35.3