Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
MIME-Version: 1.0
References: <20210324112503.623833-1-elver@google.com> <20210324112503.623833-7-elver@google.com>
 <YFxGb+QHEumZB6G8@elver.google.com> <YGHC7V3bbCxhRWTK@hirez.programming.kicks-ass.net>
 <20210329142705.GA24849@redhat.com>
In-Reply-To: <20210329142705.GA24849@redhat.com>
From:   Marco Elver <elver@google.com>
Date:   Mon, 29 Mar 2021 20:22:34 +0200
Message-ID: <CANpmjNN=dpMmanU1mzigUscZQ6_Bx6u4u5mS4Ukhy0PTiexgDA@mail.gmail.com>
Subject: Re: [PATCH v3 06/11] perf: Add support for SIGTRAP on perf events
To:     Oleg Nesterov <oleg@redhat.com>
Cc:     Peter Zijlstra <peterz@infradead.org>,
        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>
Content-Type: text/plain; charset="UTF-8"
Precedence: bulk

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