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Subject: Re: [PATCH] fs: process fput task_work with TWA_SIGNAL
To:     Al Viro <viro@zeniv.linux.org.uk>
Cc:     linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Oleg Nesterov <oleg@redhat.com>,
        Song Liu <songliubraving@fb.com>
References: <d6ddf6c2-3789-2e10-ba71-668cba03eb35@kernel.dk>
 <20210108052651.GM3579531@ZenIV.linux.org.uk>
 <20210108064639.GN3579531@ZenIV.linux.org.uk>
 <245fba32-76cc-c4e1-6007-0b1f8a22a86b@kernel.dk>
 <20210108155807.GQ3579531@ZenIV.linux.org.uk>
From:   Jens Axboe <axboe@kernel.dk>
Message-ID: <41e33492-7b01-6801-cbb1-78ecef0c9fc0@kernel.dk>
Date:   Fri, 8 Jan 2021 09:10:23 -0700
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On 1/8/21 8:58 AM, Al Viro wrote:
> On Fri, Jan 08, 2021 at 08:13:25AM -0700, Jens Axboe wrote:
>>> Anyway, bedtime for me; right now it looks like at least for task ==
>>> current we always want TWA_SIGNAL.  I'll look into that more tomorrow
>>> when I get up, but so far it smells like switching everything to
>>> TWA_SIGNAL would be the right thing to do, if not going back to bool
>>> notify for task_work_add()...
>>
>> Before the change, the fact that we ran task_work off get_signal() and
>> thus processed even non-notify work in that path was a bit of a mess,
>> imho. If you have work that needs processing now, in the same manner as
>> signals, then you really should be using TWA_SIGNAL. For this pipe case,
>> and I'd need to setup and reproduce it again, the task must have a
>> signal pending and that would have previously caused the task_work to
>> run, and now it does not. TWA_RESUME technically didn't change its
>> behavior, it's still the same notification type, we just don't run
>> task_work unconditionally (regardless of notification type) from
>> get_signal().
> 
> It sure as hell did change behaviour.  Think of the effect of getting
> hit with SIGSTOP.  That's what that "bit of a mess" had been about.
> Work done now vs. possibly several days later when SIGCONT finally
> gets sent.
> 
>> I think the main question here is if we want to re-instate the behavior
>> of running task_work off get_signal(). I'm leaning towards not doing
>> that and ensuring that callers that DO need that are using TWA_SIGNAL.
> 
> Can you show the callers that DO NOT need it?

OK, so here's my suggestion:

1) For 5.11, we just re-instate the task_work run in get_signal(). This
   will make TWA_RESUME have the exact same behavior as before.

2) For 5.12, I'll prepare a patch that collapses TWA_RESUME and TWA_SIGNAL,
   turning it into a bool again (notify or no notify).

How does that sound?

-- 
Jens Axboe