linux-kernel - [PATCH v2 2/2] fscache: Add the missing smp_mb__after_atomic() before wake_up

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Message-Id: <20221226103309.953112-3-houtao@huaweicloud.com>
Date:   Mon, 26 Dec 2022 18:33:09 +0800
From:   Hou Tao <houtao@...weicloud.com>
To:     linux-cachefs@...hat.com
Cc:     David Howells <dhowells@...hat.com>,
        Jeff Layton <jlayton@...nel.org>, linux-erofs@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org,
        Jingbo Xu <jefflexu@...ux.alibaba.com>, houtao1@...wei.com
Subject: [PATCH v2 2/2] fscache: Add the missing smp_mb__after_atomic() before wake_up_bit()

From: Hou Tao <houtao1@...wei.com>

fscache_create_volume_work() uses wake_up_bit() to wake up the processes
which are waiting for the completion of volume creation. According to
comments in wake_up_bit() and waitqueue_active(), an extra smp_mb() is
needed to guarantee the memory order between FSCACHE_VOLUME_CREATING
flag and waitqueue_active() before invoking wake_up_bit().

Considering clear_bit_unlock() before wake_up_bit() is an atomic
operation, use smp_mb__after_atomic() instead of smp_mb() to provide
such guarantee.

Signed-off-by: Hou Tao <houtao1@...wei.com>
---
 fs/fscache/volume.c | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/fs/fscache/volume.c b/fs/fscache/volume.c
index fc3dd3bc851d..734d17f404e7 100644
--- a/fs/fscache/volume.c
+++ b/fs/fscache/volume.c
@@ -281,6 +281,11 @@ static void fscache_create_volume_work(struct work_struct *work)
 				 fscache_access_acquire_volume_end);
 
 	clear_bit_unlock(FSCACHE_VOLUME_CREATING, &volume->flags);
+	/*
+	 * Paired with barrier in wait_on_bit(). Check wake_up_bit() and
+	 * waitqueue_active() for details.
+	 */
+	smp_mb__after_atomic();
 	wake_up_bit(&volume->flags, FSCACHE_VOLUME_CREATING);
 	fscache_put_volume(volume, fscache_volume_put_create_work);
 }
-- 
2.29.2