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hrtimer: Improve comments on handling priority inversion against softirq kthread

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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
f61eff83ce (hrtimer: Prepare support for PREEMPT_RT").

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

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20190820132656.GC2093@lenoir
This commit is contained in:
Frederic Weisbecker 2019-08-20 15:12:23 +02:00 committed by Thomas Gleixner
parent ec8f954a40
commit 0bee3b601b
2 changed files with 16 additions and 4 deletions
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14
kernel/time/hrtimer.c
View File

@ -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)
{

6
kernel/time/posix-timers.c
View File

@ -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)
{