linux-kernel - [tip: timers/core] posix-timers: Clarify timer_wait

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Date:   Mon, 05 Jun 2023 15:08:31 -0000
From:   "tip-bot2 for Thomas Gleixner" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Frederic Weisbecker <frederic@...nel.org>, x86@...nel.org,
        linux-kernel@...r.kernel.org
Subject: [tip: timers/core] posix-timers: Clarify timer_wait_running() comment

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@...utronix.de>
AuthorDate:    Tue, 25 Apr 2023 20:49:00 +02:00
Committer:     Thomas Gleixner <tglx@...utronix.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@...utronix.de>
Reviewed-by: Frederic Weisbecker <frederic@...nel.org>
Link: https://lore.kernel.org/r/20230425183312.985681995@linutronix.de

---
 kernel/time/posix-timers.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/kernel/time/posix-timers.c b/kernel/time/posix-timers.c
index 2c5daeb..4017533 100644
--- a/kernel/time/posix-timers.c
+++ b/kernel/time/posix-timers.c
@@ -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)