The following commit has been merged into the timers/core branch of tip: Commit-ID: 0bee3b601b77dbe7981b5474ae8758d6bf60177a Gitweb: https://git.kernel.org/tip/0bee3b601b77dbe7981b5474ae8758d6bf60177a Author: Frederic Weisbecker <frede...@kernel.org> AuthorDate: Tue, 20 Aug 2019 15:12:23 +02:00 Committer: Thomas Gleixner <t...@linutronix.de> CommitterDate: Tue, 20 Aug 2019 22:05:46 +02:00
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 f61eff83cec9 (hrtimer: Prepare support for PREEMPT_RT"). Extract the detailed informations from there and put it into comments. Signed-off-by: Frederic Weisbecker <frede...@kernel.org> Signed-off-by: Thomas Gleixner <t...@linutronix.de> Link: https://lkml.kernel.org/r/20190820132656.GC2093@lenoir --- kernel/time/hrtimer.c | 14 ++++++++++---- kernel/time/posix-timers.c | 6 ++++++ 2 files changed, 16 insertions(+), 4 deletions(-) diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c index 4991227..8333537 100644 --- a/kernel/time/hrtimer.c +++ b/kernel/time/hrtimer.c @@ -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) { diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c index 9e37783..0ec5b7a 100644 --- a/kernel/time/posix-timers.c +++ b/kernel/time/posix-timers.c @@ -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) {
