linux-ext4 - Re: possible deadlock in start_this

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Date:   Fri, 12 Feb 2021 21:58:15 +0900
From:   Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
To:     Michal Hocko <mhocko@...e.com>,
        Matthew Wilcox <willy@...radead.org>
Cc:     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 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.
You say "nobody should", but the reality is "there indeed was". There might be unnoticed others. The point of
this proposal is to make it possible to "find such unnoticed users who are manipulating PF flags on remote tasks".