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Commit ec8f954a authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

posix-timers: Use a callback for cancel synchronization on PREEMPT_RT 



Posix timer delete retry loops are affected by the same priority inversion
and live lock issues as the other timers.
    
Provide a RT specific synchronization function which keeps a reference to
the timer by holding rcu read lock to prevent the timer from being freed,
dropping the timer lock and invoking the timer specific wait function via a
new callback.
    
This does not yet cover posix CPU timers because they need more special
treatment on PREEMPT_RT.

[ This is folded into the original attempt which did not use a callback. ]

Originally-by: default avatarAnna-Maria Gleixenr <anna-maria@linutronix.de>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Reviewed-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Link: https://lkml.kernel.org/r/20190819143801.656864506@linutronix.de
parent 5d99b32a

kernel/time/alarmtimer.c

+14 −0
Original line number Diff line number Diff line
@@ -605,6 +605,19 @@ static int alarm_timer_try_to_cancel(struct k_itimer *timr) 
	return alarm_try_to_cancel(&timr->it.alarm.alarmtimer);

}



/**

 * alarm_timer_wait_running - Posix timer callback to wait for a timer

 * @timr:	Pointer to the posixtimer data struct

 *

 * Called from the core code when timer cancel detected that the callback

 * is running. @timr is unlocked and rcu read lock is held to prevent it

 * from being freed.

 */

static void alarm_timer_wait_running(struct k_itimer *timr)

{

	hrtimer_cancel_wait_running(&timr->it.alarm.alarmtimer.timer);

}



/**

 * alarm_timer_arm - Posix timer callback to arm a timer

 * @timr:	Pointer to the posixtimer data struct
@@ -834,6 +847,7 @@ const struct k_clock alarm_clock = { 
	.timer_forward		= alarm_timer_forward,

	.timer_remaining	= alarm_timer_remaining,

	.timer_try_to_cancel	= alarm_timer_try_to_cancel,

	.timer_wait_running	= alarm_timer_wait_running,

	.nsleep			= alarm_timer_nsleep,

};

#endif /* CONFIG_POSIX_TIMERS */

kernel/time/posix-timers.c

+17 −1
Original line number Diff line number Diff line
@@ -805,13 +805,25 @@ static int common_hrtimer_try_to_cancel(struct k_itimer *timr) 
	return hrtimer_try_to_cancel(&timr->it.real.timer);

}



static void common_timer_wait_running(struct k_itimer *timer)

{

	hrtimer_cancel_wait_running(&timer->it.real.timer);

}



static struct k_itimer *timer_wait_running(struct k_itimer *timer,

					   unsigned long *flags)

{

	const struct k_clock *kc = READ_ONCE(timer->kclock);

	timer_t timer_id = READ_ONCE(timer->it_id);



	/* Prevent kfree(timer) after dropping the lock */

	rcu_read_lock();

	unlock_timer(timer, *flags);

	cpu_relax();



	if (!WARN_ON_ONCE(!kc->timer_wait_running))

		kc->timer_wait_running(timer);



	rcu_read_unlock();

	/* Relock the timer. It might be not longer hashed. */

	return lock_timer(timer_id, flags);

}
@@ -1255,6 +1267,7 @@ static const struct k_clock clock_realtime = { 
	.timer_forward		= common_hrtimer_forward,

	.timer_remaining	= common_hrtimer_remaining,

	.timer_try_to_cancel	= common_hrtimer_try_to_cancel,

	.timer_wait_running	= common_timer_wait_running,

	.timer_arm		= common_hrtimer_arm,

};
@@ -1270,6 +1283,7 @@ static const struct k_clock clock_monotonic = { 
	.timer_forward		= common_hrtimer_forward,

	.timer_remaining	= common_hrtimer_remaining,

	.timer_try_to_cancel	= common_hrtimer_try_to_cancel,

	.timer_wait_running	= common_timer_wait_running,

	.timer_arm		= common_hrtimer_arm,

};
@@ -1300,6 +1314,7 @@ static const struct k_clock clock_tai = { 
	.timer_forward		= common_hrtimer_forward,

	.timer_remaining	= common_hrtimer_remaining,

	.timer_try_to_cancel	= common_hrtimer_try_to_cancel,

	.timer_wait_running	= common_timer_wait_running,

	.timer_arm		= common_hrtimer_arm,

};
@@ -1315,6 +1330,7 @@ static const struct k_clock clock_boottime = { 
	.timer_forward		= common_hrtimer_forward,

	.timer_remaining	= common_hrtimer_remaining,

	.timer_try_to_cancel	= common_hrtimer_try_to_cancel,

	.timer_wait_running	= common_timer_wait_running,

	.timer_arm		= common_hrtimer_arm,

};

kernel/time/posix-timers.h

+1 −0
Original line number Diff line number Diff line
@@ -24,6 +24,7 @@ struct k_clock { 
	int	(*timer_try_to_cancel)(struct k_itimer *timr);

	void	(*timer_arm)(struct k_itimer *timr, ktime_t expires,

			     bool absolute, bool sigev_none);

	void	(*timer_wait_running)(struct k_itimer *timr);

};



extern const struct k_clock clock_posix_cpu;