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    Subject[PATCH 2/4] cpuidle: Fix missing need_resched() check after rcu_idle_enter()
    Date
    Entering RCU idle mode may cause a deferred wake up of an RCU NOCB_GP
    kthread (rcuog) to be serviced.

    Usually a wake up happening while running the idle task is spotted in
    one of the need_resched() checks carefully placed within the idle loop
    that can break to the scheduler.

    Unfortunately within cpuidle the call to rcu_idle_enter() is already
    beyond the last generic need_resched() check. Some drivers may perform
    their own checks like with mwait_idle_with_hints() but many others don't
    and we may halt the CPU with a resched request unhandled, leaving the
    task hanging.

    Fix this with performing a last minute need_resched() check after
    calling rcu_idle_enter().

    Reported-by: Paul E. McKenney <paulmck@kernel.org>
    Reviewed-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    Fixes: 1098582a0f6c (sched,idle,rcu: Push rcu_idle deeper into the idle path)
    Cc: stable@vger.kernel.org
    Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
    Cc: Peter Zijlstra <peterz@infradead.org>
    Cc: Thomas Gleixner <tglx@linutronix.de>
    Cc: Ingo Molnar <mingo@kernel.org>
    Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
    ---
    drivers/cpuidle/cpuidle.c | 33 +++++++++++++++++++++++++--------
    1 file changed, 25 insertions(+), 8 deletions(-)

    diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
    index ef2ea1b12cd8..4cc1ba49ce05 100644
    --- a/drivers/cpuidle/cpuidle.c
    +++ b/drivers/cpuidle/cpuidle.c
    @@ -134,8 +134,8 @@ int cpuidle_find_deepest_state(struct cpuidle_driver *drv,
    }

    #ifdef CONFIG_SUSPEND
    -static void enter_s2idle_proper(struct cpuidle_driver *drv,
    - struct cpuidle_device *dev, int index)
    +static int enter_s2idle_proper(struct cpuidle_driver *drv,
    + struct cpuidle_device *dev, int index)
    {
    ktime_t time_start, time_end;
    struct cpuidle_state *target_state = &drv->states[index];
    @@ -151,7 +151,14 @@ static void enter_s2idle_proper(struct cpuidle_driver *drv,
    stop_critical_timings();
    if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
    rcu_idle_enter();
    - target_state->enter_s2idle(dev, drv, index);
    + /*
    + * Last need_resched() check must come after rcu_idle_enter()
    + * which may wake up RCU internal tasks.
    + */
    + if (!need_resched())
    + target_state->enter_s2idle(dev, drv, index);
    + else
    + index = -EBUSY;
    if (WARN_ON_ONCE(!irqs_disabled()))
    local_irq_disable();
    if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
    @@ -159,10 +166,13 @@ static void enter_s2idle_proper(struct cpuidle_driver *drv,
    tick_unfreeze();
    start_critical_timings();

    - time_end = ns_to_ktime(local_clock());
    + if (index > 0) {
    + time_end = ns_to_ktime(local_clock());
    + dev->states_usage[index].s2idle_time += ktime_us_delta(time_end, time_start);
    + dev->states_usage[index].s2idle_usage++;
    + }

    - dev->states_usage[index].s2idle_time += ktime_us_delta(time_end, time_start);
    - dev->states_usage[index].s2idle_usage++;
    + return index;
    }

    /**
    @@ -184,7 +194,7 @@ int cpuidle_enter_s2idle(struct cpuidle_driver *drv, struct cpuidle_device *dev)
    */
    index = find_deepest_state(drv, dev, U64_MAX, 0, true);
    if (index > 0) {
    - enter_s2idle_proper(drv, dev, index);
    + index = enter_s2idle_proper(drv, dev, index);
    local_irq_enable();
    }
    return index;
    @@ -234,7 +244,14 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
    stop_critical_timings();
    if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
    rcu_idle_enter();
    - entered_state = target_state->enter(dev, drv, index);
    + /*
    + * Last need_resched() check must come after rcu_idle_enter()
    + * which may wake up RCU internal tasks.
    + */
    + if (!need_resched())
    + entered_state = target_state->enter(dev, drv, index);
    + else
    + entered_state = -EBUSY;
    if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
    rcu_idle_exit();
    start_critical_timings();
    --
    2.25.1
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