Re: [PATCH 1/6] posix-cpu-timers: Fix rearm racing against process tick

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Jun 11, 2021 at 01:49:08PM +0200, Peter Zijlstra wrote:
> On Wed, Jun 09, 2021 at 01:54:00PM +0200, Frederic Weisbecker wrote:
> > On Fri, Jun 04, 2021 at 01:31:54PM +0200, Frederic Weisbecker wrote:
> > > Since the process wide cputime counter is started locklessly from
> > > posix_cpu_timer_rearm(), it can be concurrently stopped by operations
> > > on other timers from the same thread group, such as in the following
> > > unlucky scenario:
> > > 
> > >          CPU 0                                CPU 1
> > >          -----                                -----
> > >                                            timer_settime(TIMER B)
> > >    posix_cpu_timer_rearm(TIMER A)
> > >        cpu_clock_sample_group()
> > >            (pct->timers_active already true)
> > > 
> > >                                            handle_posix_cpu_timers()
> > >                                                check_process_timers()
> > >                                                    stop_process_timers()
> > >                                                        pct->timers_active = false
> > >        arm_timer(TIMER A)
> > > 
> > >    tick -> run_posix_cpu_timers()
> > >        // sees !pct->timers_active, ignore
> > >        // our TIMER A
> > > 
> > > Fix this with simply locking process wide cputime counting start and
> > > timer arm in the same block.
> > > 
> > > Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> > > Cc: Oleg Nesterov <oleg@redhat.com>
> > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > Cc: Peter Zijlstra (Intel) <peterz@infradead.org>
> > > Cc: Ingo Molnar <mingo@kernel.org>
> > > Cc: Eric W. Biederman <ebiederm@xmission.com>
> > 
> > Fixes: 60f2ceaa8111 ("posix-cpu-timers: Remove unnecessary locking around cpu_clock_sample_group")
> > Cc: stable@vger.kernel.org
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> 
> 
> Problem seems to be calling cpu_clock_sample_group(.start = true)
> without sighand locked. Do we want a lockdep assertion for that?

It's part of the problem. The other part is that it must be locked in the
same sequence than arm_timer(). So yes, a lockdep assertion would already be
a good indicator that something goes wrong.

Thanks.