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

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Date:   Mon, 29 Mar 2021 20:22:34 +0200
From:   Marco Elver <elver@...gle.com>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Ingo Molnar <mingo@...hat.com>, Jiri Olsa <jolsa@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Namhyung Kim <namhyung@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexander Potapenko <glider@...gle.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Arnd Bergmann <arnd@...db.de>,
        Christian Brauner <christian@...uner.io>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Jann Horn <jannh@...gle.com>, Jens Axboe <axboe@...nel.dk>,
        Matt Morehouse <mascasa@...gle.com>,
        Peter Collingbourne <pcc@...gle.com>,
        Ian Rogers <irogers@...gle.com>,
        kasan-dev <kasan-dev@...glegroups.com>,
        linux-arch <linux-arch@...r.kernel.org>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@...r.kernel.org>, Jiri Olsa <jolsa@...nel.org>
Subject: Re: [PATCH v3 06/11] perf: Add support for SIGTRAP on perf events

On Mon, 29 Mar 2021 at 16:27, Oleg Nesterov <oleg@...hat.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