[PATCH] hrtimer: Improve comments on handling priority inversion against softirq kthread

[Frederic Weisbecker]

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 where we don't even know why
there is a specific RT wait callback.

All the nice explanations can be found in the initial changelog of
f61eff83cec9cfab31fd30a2ca8856be379cdcd5
(hrtimer: Prepare support for PREEMPT_RT"). So lets extract the detailed
informations from there.

Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
---
 kernel/time/hrtimer.c      | 14 ++++++++++----
 kernel/time/posix-timers.c |  5 +++++
 2 files changed, 15 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)
 {
diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index a71c1aab071c..f6713a41e4e0 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -806,6 +806,11 @@ static int common_hrtimer_try_to_cancel(struct k_itimer *timr)
 }
 
 #ifdef CONFIG_PREEMPT_RT
+/*
+ * Prevent from priority inversion against softirq kthread in case
+ * it gets preempted while executing an htimer callback. See
+ * comments in hrtimer_cancel_wait_running.
+ */
 static struct k_itimer *timer_wait_running(struct k_itimer *timer,
 					   unsigned long *flags)
 {
-- 
2.21.0