[tip:,timers/core] posix-timers: Clarify timer_wait_running() comment
Commit Message
The following commit has been merged into the timers/core branch of tip:
Commit-ID: 07ca14d623d8b0e6c4a0d3de4ca8a8ea35a85470
Gitweb: https://git.kernel.org/tip/07ca14d623d8b0e6c4a0d3de4ca8a8ea35a85470
Author: Thomas Gleixner <tglx@linutronix.de>
AuthorDate: Tue, 25 Apr 2023 20:49:00 +02:00
Committer: Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Mon, 05 Jun 2023 17:03:37 +02:00
posix-timers: Clarify timer_wait_running() comment
Explain it better and add the CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y aspect
for completeness.
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Link: https://lore.kernel.org/r/20230425183312.985681995@linutronix.de
---
kernel/time/posix-timers.c | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)
@@ -836,10 +836,18 @@ static void common_timer_wait_running(struct k_itimer *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().
+ * On PREEMPT_RT this prevents priority inversion and a potential livelock
+ * against the ksoftirqd thread in case that ksoftirqd gets preempted while
+ * executing a hrtimer callback.
+ *
+ * See the comments in hrtimer_cancel_wait_running(). For PREEMPT_RT=n this
+ * just results in a cpu_relax().
+ *
+ * For POSIX CPU timers with CONFIG_POSIX_CPU_TIMERS_TASK_WORK=n this is
+ * just a cpu_relax(). With CONFIG_POSIX_CPU_TIMERS_TASK_WORK=y this
+ * prevents spinning on an eventually scheduled out task and a livelock
+ * when the task which tries to delete or disarm the timer has preempted
+ * the task which runs the expiry in task work context.
*/
static struct k_itimer *timer_wait_running(struct k_itimer *timer,
unsigned long *flags)