From: Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Jens Axboe <axboe@kernel.dk>, Jan Kara <jack@suse.cz>,
syzbot <syzbot+4a7438e774b21ddd8eca@syzkaller.appspotmail.com>,
syzkaller-bugs <syzkaller-bugs@googlegroups.com>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
LKML <linux-kernel@vger.kernel.org>,
Al Viro <viro@zeniv.linux.org.uk>, Tejun Heo <tj@kernel.org>,
Dave Chinner <david@fromorbit.com>,
linux-block@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: general protection fault in wb_workfn (2)
Date: Sat, 9 Jun 2018 14:30:41 +0900 [thread overview]
Message-ID: <2b437c6f-3e10-3d83-bdf3-82075d3eaa1a@i-love.sakura.ne.jp> (raw)
In-Reply-To: <CACT4Y+Z4i=bD49cb4pJ5zOMxVu675NT8wCNk+c2n2XJVOXG2bg@mail.gmail.com>
Dmitry Vyukov wrote:
> Here we go:
Great. Thank you.
>
> [ 2853.033175] WARNING: wb_workfn: device is NULL
> [ 2853.034709] wb->state=2
>
It is surprising that wb->state == WB_shutting_down .
WB_shutting_down is set by only wb_shutdown() and is always cleared
before leaving wb_shutdown(). This means that someone was calling
wb_shutdown() on this wb object. And bdi->dev == NULL means that
bdi_unregister() already did bdi->dev = NULL while someone was still
inside wb_shutdown().
Since we call wb_shutdown() from bdi_unregister() for each wb object
on this bdi object, this should not happen. But since "INFO: task hung
in wb_shutdown (2)" found that it is possible that wb_shutdown() is
concurrently called on the same wb object, there might be something
complicated concurrency.
Well, is it really true that "we call wb_shutdown() from bdi_unregister()
for each wb object on this bdi object"? It seems it is not always true...
While cgwb_bdi_unregister() from bdi_unregister() calls wb_shutdown() on
each wb object reachable from bdi->wb_list, wb_shutdown() firstly calls
list_del_rcu(&wb->bdi_node) (which was added by
list_add_tail_rcu(&wb->bdi_node, &bdi->wb_list) from cgwb_create()) and
then starts waiting for that wb object by calling
mod_delayed_work()/flush_delayed_work() and then clears WB_shutting_down.
Then, it is possible that cgwb_bdi_unregister() from calls wb_shutdown()
fails to find a wb object which already passed list_del_rcu() from
wb_shutdown(), and cgwb_bdi_unregister() can return without waiting for
somebody who is waiting inside wb_shutdown(). Hence, allows doing
bdi->dev = NULL before a wb object which somebody is waiting inside
wb_shutdown() completes wb_workfn(), and NULL pointer dereference...
next prev parent reply other threads:[~2018-06-09 5:33 UTC|newest]
Thread overview: 38+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-05-26 9:15 general protection fault in wb_workfn (2) syzbot
2018-05-27 0:47 ` Tetsuo Handa
2018-05-27 2:21 ` [PATCH] bdi: Fix another oops in wb_workfn() Tetsuo Handa
2018-05-27 2:36 ` Tejun Heo
2018-05-28 13:35 ` general protection fault in wb_workfn (2) Jan Kara
2018-05-30 16:00 ` Tetsuo Handa
2018-05-31 11:42 ` Jan Kara
2018-05-31 13:19 ` Tetsuo Handa
2018-05-31 13:42 ` Jan Kara
2018-05-31 16:56 ` Jens Axboe
2018-06-05 13:45 ` Tetsuo Handa
2018-06-07 18:46 ` Dmitry Vyukov
2018-06-08 2:31 ` Tetsuo Handa
2018-06-08 14:45 ` Dmitry Vyukov
2018-06-08 15:16 ` Dmitry Vyukov
2018-06-08 16:53 ` Dmitry Vyukov
2018-06-08 17:14 ` Dmitry Vyukov
2018-06-09 5:30 ` Tetsuo Handa [this message]
2018-06-09 14:00 ` [PATCH] bdi: Fix another oops in wb_workfn() Tetsuo Handa
2018-06-11 9:12 ` Jan Kara
2018-06-11 16:01 ` Tejun Heo
2018-06-11 16:29 ` Jan Kara
2018-06-11 17:20 ` Tejun Heo
2018-06-12 15:57 ` Jan Kara
2018-06-13 10:43 ` Tetsuo Handa
2018-06-13 11:51 ` Tetsuo Handa
2018-06-13 14:06 ` Linus Torvalds
2018-06-13 14:46 ` Jan Kara
2018-06-13 14:55 ` Linus Torvalds
2018-06-13 16:20 ` Tetsuo Handa
2018-06-13 16:25 ` Linus Torvalds
2018-06-13 16:45 ` Jan Kara
2018-06-13 21:04 ` Tetsuo Handa
2018-06-14 10:11 ` Jan Kara
2018-06-13 14:33 ` Tejun Heo
2018-06-15 12:06 ` Jan Kara
2018-06-18 12:27 ` Jan Kara
2018-06-01 2:30 ` general protection fault in wb_workfn (2) Dave Chinner
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