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Date:   Wed, 9 Mar 2022 16:26:49 -0700
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Subject: Re: [PATCH 07/13] task_work: Introduce task_work_pending
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To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     linux-kernel@vger.kernel.org,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Alexey Gladkov <legion@kernel.org>,
        Kyle Huey <me@kylehuey.com>, Oleg Nesterov <oleg@redhat.com>,
        Kees Cook <keescook@chromium.org>,
        Al Viro <viro@ZenIV.linux.org.uk>
References: <87o82gdlu9.fsf_-_@email.froward.int.ebiederm.org>
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 <01459886-2393-665a-43b1-70082ceace0c@kernel.dk>
 <87cziubtfu.fsf@email.froward.int.ebiederm.org>
From:   Jens Axboe <axboe@kernel.dk>
In-Reply-To: <87cziubtfu.fsf@email.froward.int.ebiederm.org>
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On 3/9/22 4:24 PM, Eric W. Biederman wrote:
> Jens Axboe <axboe@kernel.dk> writes:
> 
>> On 3/9/22 9:24 AM, Eric W. Biederman wrote:
>>> diff --git a/include/linux/task_work.h b/include/linux/task_work.h
>>> index 5b8a93f288bb..897494b597ba 100644
>>> --- a/include/linux/task_work.h
>>> +++ b/include/linux/task_work.h
>>> @@ -19,6 +19,11 @@ enum task_work_notify_mode {
>>>  	TWA_SIGNAL,
>>>  };
>>>  
>>> +static inline bool task_work_pending(struct task_struct *task)
>>> +{
>>> +	return READ_ONCE(task->task_works);
>>> +}
>>> +
>>
>> Most of the checks for this is current, do we need READ_ONCE here?
> 
> There is a non-current use in fs/io_uring in __io_uring_show_fdinfo
> and another in task_work_cancel_match.
> 
> Beyond that there are quite a few writes that are not at all from
> current so even on current task->task_works can change if you look
> twice.
> 
> So if only to keep it from making unwarranted assumptions I think
> READ_ONCE makes sense.
> 
> Given that READ_ONCE is practically free I don't see where there is
> any harm in using it to document the kind of code we expect the compiler
> to generate.
> 
> Looking a second time I see all of the other reads of task->task_works
> are already READ_ONCE in kernel/task_work.c, so really I think if we
> don't want READ_ONCE we need a big fat comment about why it is safe
> in a check like task_work_pending and while it is needed everywhere
> else.  At the moment I am not smart enough to write that comment.
> 
> I will see about adding this bit of discussion in the commit comment to
> make it a little clearer why I am introducing READ_ONCE.

Fair enough, and doesn't warrant a current_tw_pending() helper in that
case either.

-- 
Jens Axboe