linux-kernel - Re: [PATCH] fs: process fput task_work with TWA

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Message-ID: <20210108155807.GQ3579531@ZenIV.linux.org.uk>
Date:   Fri, 8 Jan 2021 15:58:07 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Jens Axboe <axboe@...nel.dk>
Cc:     linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Song Liu <songliubraving@...com>
Subject: Re: [PATCH] fs: process fput task_work with TWA_SIGNAL

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?