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

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Date:   Mon, 11 Jun 2018 09:30:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will.deacon@....com>,
        syzbot <syzbot+7b2866454055e43c21e5@...kaller.appspotmail.com>,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        syzkaller-bugs@...glegroups.com, viro@...iv.linux.org.uk
Subject: Re: INFO: task hung in __sb_start_write

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.