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

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