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

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Message-ID: <20210108065207.GP3579531@ZenIV.linux.org.uk>
Date:   Fri, 8 Jan 2021 06:52:07 +0000
From:   Al Viro <viro@...iv.linux.org.uk>
To:     Sedat Dilek <sedat.dilek@...il.com>
Cc:     Jens Axboe <axboe@...nel.dk>,
        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 07:21:52AM +0100, Sedat Dilek wrote:
> On Fri, Jan 8, 2021 at 6:30 AM Al Viro <viro@...iv.linux.org.uk> wrote:
> >
> > On Tue, Jan 05, 2021 at 11:29:11AM -0700, Jens Axboe wrote:
> > > Song reported a boot regression in a kvm image with 5.11-rc, and bisected
> > > it down to the below patch. Debugging this issue, turns out that the boot
> > > stalled when a task is waiting on a pipe being released. As we no longer
> > > run task_work from get_signal() unless it's queued with TWA_SIGNAL, the
> > > task goes idle without running the task_work. This prevents ->release()
> > > from being called on the pipe, which another boot task is waiting on.
> > >
> > > Use TWA_SIGNAL for the file fput work to ensure it's run before the task
> > > goes idle.
> > >
> > > Fixes: 98b89b649fce ("signal: kill JOBCTL_TASK_WORK")
> > > Reported-by: Song Liu <songliubraving@...com>
> > > Signed-off-by: Jens Axboe <axboe@...nel.dk>
> > >
> > > ---
> > >
> > > The other alternative here is obviously to re-instate the:
> > >
> > > if (unlikely(current->task_works))
> > >       task_work_run();
> > >
> > > in get_signal() that we had before this change. Might be safer in case
> > > there are other cases that need to ensure the work is run in a timely
> > > fashion, though I do think it's cleaner to long term to correctly mark
> > > task_work with the needed notification type. Comments welcome...
> >
> > Interesting...  I think I've missed the discussion of that thing; could
> > you forward the relevant thread my way or give an archive link to it?
> 
> See [1].
> 
> - Sedat -
> 
> [1] https://marc.info/?t=160987156600001&r=1&w=2

Wait, that's this very thread, starting with the posting I'd been replying
to.  Really confused now...  Was that a private bug report and an equally
private discussion?  That's what I wanted to see...

Anyway, I'm more than half-asleep right now; will get back to that in the
morning.