linux-ext4 - Re: possible deadlock in start_this

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Date:   Fri, 12 Feb 2021 22:34:23 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Michal Hocko <mhocko@...e.com>
Cc:     Matthew Wilcox <willy@...radead.org>, Jan Kara <jack@...e.cz>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        syzbot <syzbot+bfdded10ab7dcd7507ae@...kaller.appspotmail.com>,
        Jan Kara <jack@...e.com>, linux-ext4@...r.kernel.org,
        LKML <linux-kernel@...r.kernel.org>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        "Theodore Ts'o" <tytso@....edu>, Linux-MM <linux-mm@...ck.org>
Subject: Re: possible deadlock in start_this_handle (2)

On 2021/02/12 22:12, Michal Hocko wrote:
> On Fri 12-02-21 21:58:15, Tetsuo Handa wrote:
>> On 2021/02/12 21:30, Michal Hocko wrote:
>>> On Fri 12-02-21 12:22:07, Matthew Wilcox wrote:
>>>> On Fri, Feb 12, 2021 at 08:18:11PM +0900, Tetsuo Handa wrote:
>>>>> On 2021/02/12 1:41, Michal Hocko wrote:
>>>>>> But I suspect we have drifted away from the original issue. I thought
>>>>>> that a simple check would help us narrow down this particular case and
>>>>>> somebody messing up from the IRQ context didn't sound like a completely
>>>>>> off.
>>>>>>
>>>>>
>>>>>  From my experience at https://lkml.kernel.org/r/201409192053.IHJ35462.JLOMOSOFFVtQFH@I-love.SAKURA.ne.jp ,
>>>>> I think we can replace direct PF_* manipulation with macros which do not receive "struct task_struct *" argument.
>>>>> Since TASK_PFA_TEST()/TASK_PFA_SET()/TASK_PFA_CLEAR() are for manipulating PFA_* flags on a remote thread, we can
>>>>> define similar ones for manipulating PF_* flags on current thread. Then, auditing dangerous users becomes easier.
>>>>
>>>> No, nobody is manipulating another task's GFP flags.
>>>
>>> Agreed. And nobody should be manipulating PF flags on remote tasks
>>> either.
>>>
>>
>> No. You are misunderstanding. The bug report above is an example of manipulating PF flags on remote tasks.
> 
> Could you be more specific? I do not remember there was any theory that
> somebody is manipulating flags on a remote task. A very vague theory was
> that an interrupt context might be doing that on the _current_ context
> but even that is not based on any real evidence. It is a pure
> speculation.
> 

Please read the link above. The report is an example of manipulating PF flags on a remote task.
You are thinking interrupt context as the only possible culprit, but you should also think
concurrent access as the other possible culprit.