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

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Date:   Fri, 8 Jan 2021 18:05:35 +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 09:26:40AM -0700, Jens Axboe wrote:
> >> 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?
> 
> Attached the patches - #1 is proposed for 5.11 to fix the current issue,
> and then 2-4 can get queued for 5.12 to totally remove the difference
> between TWA_RESUME and TWA_SIGNAL.
> 
> Totally untested, but pretty straight forward.

	Umm...  I'm looking at the callers of get_signal() and I really wonder
how your support for TIF_NOTIFY_SIGNAL interacts with saved sigmask handling
by various do_signal() (calls of restore_saved_sigmask()).  Could you give
pointers to relevant discussion or a braindump on the same?  I realize that
it had been months ago, but...

	Do we even need restore_saved_sigmask_unless() now?  Could
set_user_sigmask() simply set TIF_NOTIFY_SIGNAL?  Oleg, could you comment
on that?

	Another fun question is how does that thing interact with
single-stepping logics; it's been about 8 years since I looked into
those horrors, but they used to be bloody awful...

	What I'm trying to figure out is how costly TIF_NOTIFY_SIGNAL is
on the work execution side; task_work_add() side is cheap enough, it's
delivery that is interesting.