path: root/kernel/time/hrtimer.c
diff options
authorFrederic Weisbecker <frederic@kernel.org>2019-08-20 15:12:23 +0200
committerThomas Gleixner <tglx@linutronix.de>2019-08-20 22:05:46 +0200
commit0bee3b601b77dbe7981b5474ae8758d6bf60177a (patch)
treebb5c920be193be0a68973d54969868b680e8ab9f /kernel/time/hrtimer.c
parentposix-timers: Use a callback for cancel synchronization on PREEMPT_RT (diff)
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 <frederic@kernel.org> Signed-off-by: Thomas Gleixner <tglx@linutronix.de> Link: https://lkml.kernel.org/r/20190820132656.GC2093@lenoir
Diffstat (limited to 'kernel/time/hrtimer.c')
1 files changed, 10 insertions, 4 deletions
diff --git a/kernel/time/hrtimer.c b/kernel/time/hrtimer.c
index 499122752649..833353732554 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)