linux-kernel - [PATCH] eventfd: avoid unnecessary wakeups in eventfd

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Date:   Thu, 13 Jul 2023 00:42:32 +0800
From:   wenyang.linux@...mail.com
To:     Alexander Viro <viro@...iv.linux.org.uk>,
        Jens Axboe <axboe@...nel.dk>,
        Christian Brauner <brauner@...nel.org>
Cc:     Wen Yang <wenyang.linux@...mail.com>,
        Christoph Hellwig <hch@....de>, Dylan Yudaken <dylany@...com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Matthew Wilcox <willy@...radead.org>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: [PATCH] eventfd: avoid unnecessary wakeups in eventfd_write()

From: Wen Yang <wenyang.linux@...mail.com>

In eventfd_write(), when ucnt is 0 and ctx->count is also 0,
current->in_eventfd will be set to 1, which may affect eventfd_signal(),
and unnecessary wakeups will also be performed.

Fix this issue by ensuring that ctx->count is not zero.

Signed-off-by: Wen Yang <wenyang.linux@...mail.com>
Cc: Alexander Viro <viro@...iv.linux.org.uk>
Cc: Jens Axboe <axboe@...nel.dk>
Cc: Christian Brauner <brauner@...nel.org>
Cc: Christoph Hellwig <hch@....de>
Cc: Dylan Yudaken <dylany@...com>
Cc: David Woodhouse <dwmw@...zon.co.uk>
Cc: Matthew Wilcox <willy@...radead.org>
Cc: linux-fsdevel@...r.kernel.org
Cc: linux-kernel@...r.kernel.org
---
 fs/eventfd.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/fs/eventfd.c b/fs/eventfd.c
index 33a918f9566c..254b18ff0e00 100644
--- a/fs/eventfd.c
+++ b/fs/eventfd.c
@@ -281,10 +281,12 @@ static ssize_t eventfd_write(struct file *file, const char __user *buf, size_t c
 	}
 	if (likely(res > 0)) {
 		ctx->count += ucnt;
-		current->in_eventfd = 1;
-		if (waitqueue_active(&ctx->wqh))
-			wake_up_locked_poll(&ctx->wqh, EPOLLIN);
-		current->in_eventfd = 0;
+		if (ctx->count) {
+			current->in_eventfd = 1;
+			if (waitqueue_active(&ctx->wqh))
+				wake_up_locked_poll(&ctx->wqh, EPOLLIN);
+			current->in_eventfd = 0;
+		}
 	}
 	spin_unlock_irq(&ctx->wqh.lock);
 
-- 
2.25.1