Re: [PATCH] sched: __fatal_signal_pending() should also check PF_EXITING

Serge E. Hallyn
2022-07-21T01:55:18Z

On Wed, Jul 20, 2022 at 02:58:42PM -0600, Tycho Andersen wrote:
> On Wed, Jul 20, 2022 at 10:03:28AM -0500, Serge E. Hallyn wrote:
> > On Wed, Jul 13, 2022 at 11:53:05AM -0600, Tycho Andersen wrote:
> > > The wait_* code uses signal_pending_state() to test whether a thread has
> > > been interrupted, which ultimately uses __fatal_signal_pending() to detect
> > > if there is a fatal signal.
> > >
> > > When a pid ns dies, it does:
> > >
> > > group_send_sig_info(SIGKILL, SEND_SIG_PRIV, task, PIDTYPE_MAX);
> > >
> > > for all the tasks in the pid ns. That calls through:
> > >
> > > group_send_sig_info() ->
> > > do_send_sig_info() ->
> > > send_signal_locked() ->
> > > __send_signal_locked()
> > >
> > > which does:
> > >
> > > pending = (type != PIDTYPE_PID) ? &t->signal->shared_pending : &t->pending;
> > >
> > > which puts sigkill in the set of shared signals, but not the individual
> > > pending ones. When complete_signal() is called at the end of
> > > __send_signal_locked(), if the task already had PF_EXITING (i.e. was
> > > already waiting on something in its fd closing path like a fuse flush),
> > > complete_signal() will not wake up the thread, since wants_signal() checks
> > > PF_EXITING before testing for SIGKILL.
> > >
> > > If tasks are stuck in a killable wait (e.g. a fuse flush operation), they
> > > won't see this shared signal, and will hang forever, since TIF_SIGPENDING
> > > is set, but the fatal signal can't be detected. So, let's also look for
> > > PF_EXITING in __fatal_signal_pending().
> > >
> > > Signed-off-by: Tycho Andersen <tycho@tycho.pizza>
> >
> > Cool, thanks for nailing this down!
> >
> > I assume you've been running this on some boxes with no weird effects?
>
> Yes, but I haven't tested all the paths.
>
> > > ---
> > > include/linux/sched/signal.h | 3 ++-
> > > 1 file changed, 2 insertions(+), 1 deletion(-)
> > >
> > > diff --git a/include/linux/sched/signal.h b/include/linux/sched/signal.h
> > > index cafbe03eed01..c20b7e1d89ef 100644
> > > --- a/include/linux/sched/signal.h
> > > +++ b/include/linux/sched/signal.h
> > > @@ -402,7 +402,8 @@ static inline int signal_pending(struct task_struct *p)
> > >
> > > static inline int __fatal_signal_pending(struct task_struct *p)
> > > {
> > > - return unlikely(sigismember(&p->pending.signal, SIGKILL));
> > > + return unlikely(sigismember(&p->pending.signal, SIGKILL) ||
> > > + p->flags & PF_EXITING);
> >
> > Looking around at the callers this does seem safe, but the name does
> > now seem misleading. Should this be renamed to something like
> > exiting_or_fatal_signal_pending()?
>
> This is why I like my original patch better: it is just expanding the
> set of signals to include the shared signals, which are indeed still
> fatal pending signals for the task. I don't really understand Eric's
> argument about kernel threads ignoring SIGKILL, since kernel threads

Oh - I didn't either - checking the sigkill in shared signals *seems*
legit if they can be put there - but since you posted the new patch I
assumed his reasoning was clear to you. I know Eric's busy, cc:ing Oleg
for his interpretation too.

> can still ignore SIGKILL just fine after this patch.
>
> But yes, assuming Eric is ok with this venison. I can send a v2 with
> the name change as you suggest.
>
> Thanks for looking.
>
> Tycho