linux-kernel - Re: [PATCH] sched: __fatal_signal_pending() should also check PF

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Date:   Fri, 29 Jul 2022 10:48:51 -0600
From:   Tycho Andersen <tycho@...ho.pizza>
To:     "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Oleg Nesterov <oleg@...hat.com>,
        "Serge E. Hallyn" <serge@...lyn.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: __fatal_signal_pending() should also check
 PF_EXITING

On Fri, Jul 29, 2022 at 11:15:28AM -0500, Eric W. Biederman wrote:
> Tycho Andersen <tycho@...ho.pizza> writes:
> 
> > On Fri, Jul 29, 2022 at 12:04:17AM -0500, Eric W. Biederman wrote:
> >> Tycho Andersen <tycho@...ho.pizza> writes:
> >> 
> >> > On Thu, Jul 28, 2022 at 11:12:20AM +0200, Oleg Nesterov wrote:
> >
> >> >> Finally. if fuse_flush() wants __fatal_signal_pending() == T when the
> >> >> caller exits, perhaps it can do it itself? Something like
> >> >> 
> >> >> 	if (current->flags & PF_EXITING) {
> >> >> 		spin_lock_irq(siglock);
> >> >> 		set_thread_flag(TIF_SIGPENDING);
> >> >> 		sigaddset(&current->pending.signal, SIGKILL);
> >> >> 		spin_unlock_irq(siglock);
> >> >> 	}
> >> >> 
> >> >> Sure, this is ugly as hell. But perhaps this can serve as a workaround?
> >> >
> >> > or even just
> >> >
> >> >     if (current->flags & PF_EXITING)
> >> >         return;
> >> >
> >> > since we don't have anyone to send the result of the flush to anyway.
> >> > If we don't end up converging on a fix here, I'll just send that
> >> > patch. Thanks for the suggestion.
> >> 
> >> If that was limited to the case you care about that would be reasonable.
> >> 
> >> That will have an effect on any time a process that opens files on a
> >> fuse filesystem exits and depends upon the exit path to close it's file
> >> descriptors to the fuse filesystem.
> >> 
> >> 
> >> I do see a plausible solution along those lines.
> >> 
> >> In fuse_flush instead of using fuse_simple_request call an equivalent
> >> function that when PF_EXITING is true skips calling request_wait_answer.
> >> Or perhaps when PF_EXITING is set uses schedule_work to call the
> >> request_wait_answer.
> >
> > I don't see why this is any different than what I proposed. It changes
> > the semantics to flush happening out-of-order with task exit, instead
> > of strictly before, which you point out might be a problem. What am I
> > missing?
> 
> What you proposed skips the flush operation entirely.

Sorry, I wasn't clear. I was thinking of roughly similar to what you
were, returning from request_wait_answer() early if we have
PF_EXITING. Sounds like we agree that it shouldn't be an issue. I'll
give it a test and send out a patch Monday.

Tycho