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Date:   Tue, 30 Mar 2021 09:04:36 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Marco Elver <elver@google.com>
Cc:     Oleg Nesterov <oleg@redhat.com>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Jiri Olsa <jolsa@redhat.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Alexander Potapenko <glider@google.com>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Arnd Bergmann <arnd@arndb.de>,
        Christian Brauner <christian@brauner.io>,
        Dmitry Vyukov <dvyukov@google.com>,
        Jann Horn <jannh@google.com>, Jens Axboe <axboe@kernel.dk>,
        Matt Morehouse <mascasa@google.com>,
        Peter Collingbourne <pcc@google.com>,
        Ian Rogers <irogers@google.com>,
        kasan-dev <kasan-dev@googlegroups.com>,
        linux-arch <linux-arch@vger.kernel.org>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>,
        "open list:KERNEL SELFTEST FRAMEWORK" 
        <linux-kselftest@vger.kernel.org>, Jiri Olsa <jolsa@kernel.org>
Subject: Re: [PATCH v3 06/11] perf: Add support for SIGTRAP on perf events
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On Mon, Mar 29, 2021 at 04:32:18PM +0200, Marco Elver wrote:
> 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.
> 
> Thanks for confirming. I'll switch to just checking PF_EXITING
> (PF_EXISTING does not exist :-)).

Indeed! Typing be hard :-)