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posix-timers: Clarify timer_wait_running() comment 

Explain it better and add the CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y aspect
for completeness.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20230425183312.985681995@linutronix.de
This commit is contained in:
Thomas Gleixner 2023-04-25 20:49:00 +02:00
parent 8ce8849dd1
commit 7d99090266
1 changed files with 12 additions and 4 deletions
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16
kernel/time/posix-timers.c
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@ -836,10 +836,18 @@ static void common_timer_wait_running(struct k_itimer *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().
* On PREEMPT_RT this prevents priority inversion and a potential livelock
* against the ksoftirqd thread in case that ksoftirqd gets preempted while
* executing a hrtimer callback.
*
* See the comments in hrtimer_cancel_wait_running(). For PREEMPT_RT=n this
* just results in a cpu_relax().
*
* For POSIX CPU timers with CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n this is
* just a cpu_relax(). With CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y this
* prevents spinning on an eventually scheduled out task and a livelock
* when the task which tries to delete or disarm the timer has preempted
* the task which runs the expiry in task work context.
*/
static struct k_itimer *timer_wait_running(struct k_itimer *timer,
unsigned long *flags)