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From: tip-bot2 for Frederic Weisbecker <tip-bot2@linutronix.de>
To: linux-tip-commits@vger.kernel.org
Cc: linux-kernel@vger.kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Frederic Weisbecker <frederic@kernel.org>
Subject: [tip: timers/core] hrtimer: Improve comments on handling priority inversion against softirq kthread
Date: Fri, 23 Aug 2019 02:12:17 -0000
Message-ID: <156652633741.11664.5398841645445804148.tip-bot2@tip-bot2> (raw)
In-Reply-To: <20190820132656.GC2093@lenoir>

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@kernel.org>
AuthorDate:    Tue, 20 Aug 2019 15:12:23 +02:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
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@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: 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)
 {

  reply index

Thread overview: 61+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-07-26 18:30 [patch 00/12] (hr)timers: Prepare for PREEMPT_RT support Thomas Gleixner
2019-07-26 18:30 ` [patch 01/12] hrtimer: Remove task argument from hrtimer_init_sleeper() Thomas Gleixner
2019-07-26 19:57   ` Steven Rostedt
2019-07-26 20:01     ` Thomas Gleixner
2019-07-30 22:07   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2019-07-26 18:30 ` [patch 02/12] hrtimer: Consolidate hrtimer_init() + hrtimer_init_sleeper() calls Thomas Gleixner
2019-07-30 22:08   ` [tip:timers/core] " tip-bot for Sebastian Andrzej Siewior
2019-08-01 15:49   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 03/12] hrtimer: Introduce HARD expiry mode Thomas Gleixner
2019-07-30 22:10   ` [tip:timers/core] " tip-bot for Sebastian Andrzej Siewior
2019-08-01 15:52   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 04/12] sched: Mark hrtimers to expire in hard interrupt context Thomas Gleixner
2019-07-30 22:11   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2019-08-01 15:53   ` tip-bot for Sebastian Andrzej Siewior
2019-08-01 18:58   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 05/12] perf/core: " Thomas Gleixner
2019-07-30 22:12   ` [tip:timers/core] " tip-bot for Thomas Gleixner
2019-08-01 15:54   ` tip-bot for Sebastian Andrzej Siewior
2019-08-01 18:59   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 06/12] watchdog: Mark watchdog_hrtimer " Thomas Gleixner
2019-07-30 22:13   ` [tip:timers/core] " tip-bot for Sebastian Andrzej Siewior
2019-08-01 15:55   ` tip-bot for Sebastian Andrzej Siewior
2019-08-01 19:00   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 07/12] KVM: LAPIC: Mark hrtimer " Thomas Gleixner
2019-07-26 19:41   ` Paolo Bonzini
2019-07-30 22:14   ` [tip:timers/core] " tip-bot for Sebastian Andrzej Siewior
2019-08-01 15:55   ` tip-bot for Sebastian Andrzej Siewior
2019-08-01 19:01   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 08/12] tick: Mark tick related hrtimers to expiry " Thomas Gleixner
2019-07-30 22:14   ` [tip:timers/core] " tip-bot for Sebastian Andrzej Siewior
2019-08-01 15:56   ` tip-bot for Sebastian Andrzej Siewior
2019-08-01 19:01   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 09/12] hrtimer: Move unmarked hrtimers to soft interrupt expiry on RT Thomas Gleixner
2019-07-30 22:15   ` [tip:timers/core] " tip-bot for Sebastian Andrzej Siewior
2019-08-01 15:57   ` tip-bot for Sebastian Andrzej Siewior
2019-08-01 19:02   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 10/12] hrtimer: Determine hard/soft expiry mode for hrtimer sleepers " Thomas Gleixner
2019-07-26 20:44   ` Steven Rostedt
2019-07-26 20:52     ` Thomas Gleixner
2019-07-26 20:56       ` Steven Rostedt
2019-07-26 21:16   ` Julia Cartwright
2019-07-26 21:30     ` Steven Rostedt
2019-07-26 21:35     ` Thomas Gleixner
2019-07-30 22:16   ` [tip:timers/core] " tip-bot for Sebastian Andrzej Siewior
2019-08-01 15:58   ` tip-bot for Sebastian Andrzej Siewior
2019-08-01 19:03   ` tip-bot for Sebastian Andrzej Siewior
2019-07-26 18:30 ` [patch 11/12] hrtimer: Prepare support for PREEMPT_RT Thomas Gleixner
2019-07-28  9:06   ` Juergen Gross
2019-07-29 15:08     ` Steven Rostedt
2019-07-29 17:30       ` Paolo Bonzini
2019-07-31  8:45         ` Juergen Gross
2019-07-30 22:17   ` [tip:timers/core] " tip-bot for Anna-Maria Gleixner
2019-08-01 15:58   ` tip-bot for Anna-Maria Gleixner
2019-08-01 19:04   ` tip-bot for Anna-Maria Gleixner
2019-08-20 13:26     ` Frederic Weisbecker
2019-08-23  2:12       ` tip-bot2 for Frederic Weisbecker [this message]
2019-07-26 18:31 ` [patch 12/12] timers: " Thomas Gleixner
2019-07-30 22:17   ` [tip:timers/core] " tip-bot for Anna-Maria Gleixner
2019-08-01 15:59   ` tip-bot for Anna-Maria Gleixner
2019-08-01 19:04   ` tip-bot for Anna-Maria Gleixner
2019-07-29 19:45 ` [patch 00/12] (hr)timers: Prepare for PREEMPT_RT support Peter Zijlstra

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