Re: [PATCH v3 06/11] perf: Add support for SIGTRAP on perf events

[Marco Elver <elver@google.com>]

On Mon, 29 Mar 2021 at 16:27, Oleg Nesterov <oleg@redhat.com> wrote:
> On 03/29, Peter Zijlstra wrote:
> >
> > On Thu, Mar 25, 2021 at 09:14:39AM +0100, Marco Elver wrote:
> > > @@ -6395,6 +6395,13 @@ static void perf_sigtrap(struct perf_event *event)
> > >  {
> > >     struct kernel_siginfo info;
> > >
> > > +   /*
> > > +    * This irq_work can race with an exiting task; bail out if sighand has
> > > +    * already been released in release_task().
> > > +    */
> > > +   if (!current->sighand)
> > > +           return;
>
> This is racy. If "current" has already passed exit_notify(), current->parent
> can do release_task() and destroy current->sighand right after the check.
>
> > Urgh.. I'm not entirely sure that check is correct, but I always forget
> > the rules with signal. It could be we ought to be testing PF_EXISTING
> > instead.
>
> Agreed, PF_EXISTING check makes more sense in any case, the exiting task
> can't receive the signal anyway.

So, per off-list discussion, it appears that I should ask to clarify:
PF_EXISTING or PF_EXITING?

It appears that PF_EXISTING is what's being suggested, whereas it has
not been mentioned anywhere, nor are its semantics clear. If it is not
simply the negation of PF_EXITING, what are its semantics? And why do
we need it in the case here (instead of something else that already
exists)?

Thanks,
-- Marco