linux-kernel - Re: INFO: task hung in __sb_start

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Message-ID: <CACT4Y+Zyf2e_K0oRJ5oRj3WYWpf-t=V=RKHYcdqujF3s9PJL_w@mail.gmail.com>
Date:   Mon, 11 Jun 2018 09:39:07 +0200
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        syzbot <syzbot+7b2866454055e43c21e5@...kaller.appspotmail.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Al Viro <viro@...iv.linux.org.uk>
Subject: Re: INFO: task hung in __sb_start_write

On Mon, Jun 11, 2018 at 9:30 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Sun, Jun 10, 2018 at 11:47:56PM +0900, Tetsuo Handa wrote:
>
>> This looks quite strange that nobody is holding percpu_rw_semaphore for
>> write but everybody is stuck trying to hold it for read. (Since there
>> is no "X locks held by ..." line without followup "#0:" line, there is
>> no possibility that somebody is in TASK_RUNNING state while holding
>> percpu_rw_semaphore for write.)
>>
>> I feel that either API has a bug or API usage is wrong.
>> Any idea for debugging this?
>
> Look at percpu_rwsem_release() and usage. The whole fs freezer thing is
> magic.

Do you mean that we froze fs? We tried to never-ever issue
ioctl(FIFREEZE) during fuzzing. Are there other ways to do this?