[tip: timers/core] hrtimer: Improve comments on handling priority inversion against softirq kthread

[tip-bot2 for Frederic Weisbecker]

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 <frederic@xxxxxxxxxx>
AuthorDate:    Tue, 20 Aug 2019 15:12:23 +02:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
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 <frederic@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Link: 20190820132656.GC2093@lenoir">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)
 {