Re: [PATCH AUTOSEL 5.11 43/44] signal: don't allow STOP on PF_IO_WORKER threads

[Sasha Levin]

On Thu, Mar 25, 2021 at 08:02:11AM -0600, Jens Axboe wrote:
> On 3/25/21 7:56 AM, Stefan Metzmacher wrote:
> > Am 25.03.21 um 14:38 schrieb Jens Axboe:
> > > On 3/25/21 6:11 AM, Stefan Metzmacher wrote:
> > > > Am 25.03.21 um 13:04 schrieb Eric W. Biederman:
> > > > > Stefan Metzmacher <metze@xxxxxxxxx> writes:
> > > > >
> > > > > > Am 25.03.21 um 12:24 schrieb Sasha Levin:
> > > > > > > From: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> > > > > > >
> > > > > > > [ Upstream commit 4db4b1a0d1779dc159f7b87feb97030ec0b12597 ]
> > > > > > >
> > > > > > > Just like we don't allow normal signals to IO threads, don't deliver a
> > > > > > > STOP to a task that has PF_IO_WORKER set. The IO threads don't take
> > > > > > > signals in general, and have no means of flushing out a stop either.
> > > > > > >
> > > > > > > Longer term, we may want to look into allowing stop of these threads,
> > > > > > > as it relates to eg process freezing. For now, this prevents a spin
> > > > > > > issue if a SIGSTOP is delivered to the parent task.
> > > > > > >
> > > > > > > Reported-by: Stefan Metzmacher <metze@xxxxxxxxx>
> > > > > > > Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
> > > > > > > Signed-off-by: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
> > > > > > > Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> > > > > > > ---
> > > > > > >  kernel/signal.c | 3 ++-
> > > > > > >  1 file changed, 2 insertions(+), 1 deletion(-)
> > > > > > >
> > > > > > > diff --git a/kernel/signal.c b/kernel/signal.c
> > > > > > > index 55526b941011..00a3840f6037 100644
> > > > > > > --- a/kernel/signal.c
> > > > > > > +++ b/kernel/signal.c
> > > > > > > @@ -288,7 +288,8 @@ bool task_set_jobctl_pending(struct task_struct *task, unsigned long mask)
> > > > > > >  			JOBCTL_STOP_SIGMASK | JOBCTL_TRAPPING));
> > > > > > >  	BUG_ON((mask & JOBCTL_TRAPPING) && !(mask & JOBCTL_PENDING_MASK));
> > > > > > >
> > > > > > > -	if (unlikely(fatal_signal_pending(task) || (task->flags & PF_EXITING)))
> > > > > > > +	if (unlikely(fatal_signal_pending(task) ||
> > > > > > > +		     (task->flags & (PF_EXITING | PF_IO_WORKER))))
> > > > > > >  		return false;
> > > > > > >
> > > > > > >  	if (mask & JOBCTL_STOP_SIGMASK)
> > > > > >
> > > > > > Again, why is this proposed for 5.11 and 5.10 already?
> > > > >
> > > > > Has the bit about the io worker kthreads been backported?
> > > > > If so this isn't horrible.  If not this is nonsense.
> > >
> > > No not yet - my plan is to do that, but not until we're 100% satisfied
> > > with it.
> >
> > Do you understand why the patches where autoselected for 5.11 and 5.10?
>
> As far as I know, selections like these (AUTOSEL) are done by some bot
> that uses whatever criteria to see if they should be applied for earlier
> revisions. I'm sure Sasha can expand on that :-)

Right, it's just heuristics that help catch commits that don't have a
stable tag but should have one.

> Hence it's reasonable to expect that sometimes it'll pick patches that
> should not go into stable, at least not just yet. It's important to
> understand that this message is just a notice that it's queued up for
> stable -rc, not that it's _in_ stable just yet. There's time to object.

Right, it's even more than that: this mail (tagged with "AUTOSEL") is a
notification that happens at least a week before the patch will go in
the stable queue.

If you think any AUTOSEL patches don't need to be backported, it's
usually enough to just quickly nack them.

--
Thanks,
Sasha