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Commit 0bee3b60 authored by Frederic Weisbecker's avatar Frederic Weisbecker Committed by Thomas Gleixner
Browse files

hrtimer: Improve comments on handling priority inversion against softirq kthread 



The handling of a priority inversion between timer cancelling and a a not
well defined possible preemption of softirq kthread is not very clear.

Especially in the posix timers side it's unclear why there is a specific RT
wait callback.

All the nice explanations can be found in the initial changelog of
f61eff83 (hrtimer: Prepare support for PREEMPT_RT").

Extract the detailed informations from there and put it into comments.

Signed-off-by: default avatarFrederic Weisbecker <frederic@kernel.org>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190820132656.GC2093@lenoir
parent ec8f954a

kernel/time/hrtimer.c

+10 −4
Original line number Diff line number Diff line
@@ -1201,10 +1201,16 @@ static void hrtimer_sync_wait_running(struct hrtimer_cpu_base *cpu_base, 
 * deletion of a timer failed because the timer callback function was

 * running.

 *

 * This prevents priority inversion, if the softirq thread on a remote CPU

 * got preempted, and it prevents a life lock when the task which tries to

 * delete a timer preempted the softirq thread running the timer callback

 * function.

 * This prevents priority inversion: if the soft irq thread is preempted

 * in the middle of a timer callback, then calling del_timer_sync() can

 * lead to two issues:

 *

 *  - If the caller is on a remote CPU then it has to spin wait for the timer

 *    handler to complete. This can result in unbound priority inversion.

 *

 *  - If the caller originates from the task which preempted the timer

 *    handler on the same CPU, then spin waiting for the timer handler to

 *    complete is never going to end.

 */

void hrtimer_cancel_wait_running(const struct hrtimer *timer)

{

kernel/time/posix-timers.c

+6 −0
Original line number Diff line number Diff line
@@ -810,6 +810,12 @@ static void common_timer_wait_running(struct k_itimer *timer) 
	hrtimer_cancel_wait_running(&timer->it.real.timer);

}



/*

 * On PREEMPT_RT this prevent priority inversion against softirq kthread in

 * case it gets preempted while executing a timer callback. See comments in

 * hrtimer_cancel_wait_running. For PREEMPT_RT=n this just results in a

 * cpu_relax().

 */

static struct k_itimer *timer_wait_running(struct k_itimer *timer,

					   unsigned long *flags)

{