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    Subject[PATCH 5.13 190/223] posix-cpu-timers: Fix rearm racing against process tick
    Date
    From: Frederic Weisbecker <frederic@kernel.org>

    commit 1a3402d93c73bf6bb4df6d7c2aac35abfc3c50e2 upstream.

    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.

    Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
    Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
    Fixes: 60f2ceaa8111 ("posix-cpu-timers: Remove unnecessary locking around cpu_clock_sample_group")
    Cc: stable@vger.kernel.org
    Cc: Oleg Nesterov <oleg@redhat.com>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Ingo Molnar <mingo@kernel.org>
    Cc: Eric W. Biederman <ebiederm@xmission.com>
    Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
    ---
    kernel/time/posix-cpu-timers.c | 10 +++++-----
    1 file changed, 5 insertions(+), 5 deletions(-)

    --- a/kernel/time/posix-cpu-timers.c
    +++ b/kernel/time/posix-cpu-timers.c
    @@ -991,6 +991,11 @@ static void posix_cpu_timer_rearm(struct
    if (!p)
    goto out;

    + /* Protect timer list r/w in arm_timer() */
    + sighand = lock_task_sighand(p, &flags);
    + if (unlikely(sighand == NULL))
    + goto out;
    +
    /*
    * Fetch the current sample and update the timer's expiry time.
    */
    @@ -1001,11 +1006,6 @@ static void posix_cpu_timer_rearm(struct

    bump_cpu_timer(timer, now);

    - /* Protect timer list r/w in arm_timer() */
    - sighand = lock_task_sighand(p, &flags);
    - if (unlikely(sighand == NULL))
    - goto out;
    -
    /*
    * Now re-arm for the new expiry time.
    */

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