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From:   "Eric W. Biederman" <ebiederm@xmission.com>
To:     Jens Axboe <axboe@kernel.dk>
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>
        <20220309162454.123006-7-ebiederm@xmission.com>
        <01459886-2393-665a-43b1-70082ceace0c@kernel.dk>
Date:   Wed, 09 Mar 2022 17:24:37 -0600
In-Reply-To: <01459886-2393-665a-43b1-70082ceace0c@kernel.dk> (Jens Axboe's
        message of "Wed, 9 Mar 2022 14:05:50 -0700")
Message-ID: <87cziubtfu.fsf@email.froward.int.ebiederm.org>
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Subject: Re: [PATCH 07/13] task_work: Introduce task_work_pending
Precedence: bulk

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.

Eric