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Date:   Wed, 9 Jun 2021 13:54:00 +0200
From:   Frederic Weisbecker <frederic@kernel.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     LKML <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        "Eric W . Biederman" <ebiederm@xmission.com>,
        Oleg Nesterov <oleg@redhat.com>, Ingo Molnar <mingo@kernel.org>
Subject: Re: [PATCH 1/6] posix-cpu-timers: Fix rearm racing against process
 tick
Message-ID: <20210609115400.GD104634@lothringen>
References: <20210604113159.26177-1-frederic@kernel.org>
 <20210604113159.26177-2-frederic@kernel.org>
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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