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Subject: Re: BUG: workqueue lockup (2)
From:   Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
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Date:   Sun, 13 May 2018 11:06:17 +0900
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Eric Biggers wrote:
> The bug that this reproducer reproduces was fixed a while ago by commit
> 966031f340185e, so I'm marking this bug report fixed by it:
> 
> #syz fix: n_tty: fix EXTPROC vs ICANON interaction with TIOCINQ (aka FIONREAD)

Nope. Commit 966031f340185edd ("n_tty: fix EXTPROC vs ICANON interaction with
TIOCINQ (aka FIONREAD)") is "Wed Dec 20 17:57:06 2017 -0800" but the last
occurrence on linux.git (commit 008464a9360e31b1 ("Merge branch 'for-linus' of
git://git.kernel.org/pub/scm/linux/kernel/git/jikos/hid")) is only a few days ago
("Wed May 9 10:49:52 2018 -1000").

> 
> Note that the error message was not always "BUG: workqueue lockup"; it was also
> sometimes like "watchdog: BUG: soft lockup - CPU#5 stuck for 22s!".
> 
> syzbot still is hitting the "BUG: workqueue lockup" error sometimes, but it must
> be for other reasons.  None has a reproducer currently.

The last occurrence on linux.git is considered as a duplicate of

  [upstream] INFO: rcu detected stall in n_tty_receive_char_special
  https://syzkaller.appspot.com/bug?id=3d7481a346958d9469bebbeb0537d5f056bdd6e8

which we already have a reproducer at
https://groups.google.com/d/msg/syzkaller-bugs/O4DbPiJZFBY/YCVPocx3AgAJ
and debug output is available at
https://groups.google.com/d/msg/syzkaller-bugs/O4DbPiJZFBY/TxQ7WS5ZAwAJ .

We are currently waiting for comments from Peter Hurley who added that code.