[PATCH] eventfd: avoid unnecessary wakeups in eventfd_write()

[wenyang.linux@foxmail.com]

From: Wen Yang <wenyang.linux@foxmail.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@foxmail.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Dylan Yudaken <dylany@fb.com>
Cc: David Woodhouse <dwmw@amazon.co.uk>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: linux-fsdevel@vger.kernel.org
Cc: linux-kernel@vger.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