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Subject: Re: INFO: task hung in wb_shutdown (2)
To:     Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
        Tejun Heo <tj@kernel.org>
References: <94eb2c05b2d83650030568cc8bd9@google.com>
Cc:     syzbot <syzbot+c0cf869505e03bdf1a24@syzkaller.appspotmail.com>,
        christophe.jaillet@wanadoo.fr, LKML <linux-kernel@vger.kernel.org>,
        linux-mm <linux-mm@kvack.org>, syzkaller-bugs@googlegroups.com,
        weiping zhang <zhangweiping@didichuxing.com>
From:   Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Message-ID: <e56c1600-8923-dd6b-d065-c2fd2a720404@I-love.SAKURA.ne.jp>
Date:   Tue, 24 Apr 2018 21:19:54 +0900
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From 39ed6be8a2c12dfe54feaa5abbc2ec46103022bf Mon Sep 17 00:00:00 2001
From: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Date: Tue, 24 Apr 2018 11:59:08 +0900
Subject: [PATCH] bdi: wake up concurrent wb_shutdown() callers.

syzbot is reporting hung tasks at wait_on_bit(WB_shutting_down) in
wb_shutdown() [1]. This might be because commit 5318ce7d46866e1d ("bdi:
Shutdown writeback on all cgwbs in cgwb_bdi_destroy()") forgot to call
wake_up_bit(WB_shutting_down) after clear_bit(WB_shutting_down).

[1] https://syzkaller.appspot.com/bug?id=b297474817af98d5796bc544e1bb806fc3da0e5e

Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Reported-by: syzbot <syzbot+c0cf869505e03bdf1a24@syzkaller.appspotmail.com>
Fixes: 5318ce7d46866e1d ("bdi: Shutdown writeback on all cgwbs in cgwb_bdi_destroy()")
Cc: Tejun Heo <tj@kernel.org>
Cc: Jan Kara <jack@suse.cz>
Cc: Jens Axboe <axboe@fb.com>
---
 mm/backing-dev.c | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/mm/backing-dev.c b/mm/backing-dev.c
index 023190c..dadac99 100644
--- a/mm/backing-dev.c
+++ b/mm/backing-dev.c
@@ -384,6 +384,8 @@ static void wb_shutdown(struct bdi_writeback *wb)
 	 */
 	smp_wmb();
 	clear_bit(WB_shutting_down, &wb->state);
+	smp_mb(); /* advised by wake_up_bit() */
+	wake_up_bit(&wb->state, WB_shutting_down);
 }
 
 static void wb_exit(struct bdi_writeback *wb)
-- 
1.8.3.1