[PATCH] hrtimer: Improve comments on handling priority inversion against softirq kthread
From: Frederic Weisbecker
Date: Tue Aug 20 2019 - 09:12:23 EST
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
(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
@@ -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
- * 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
@@ -806,6 +806,11 @@ static int common_hrtimer_try_to_cancel(struct k_itimer *timr)
+ * 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)