linux-kernel - [patch 2/7] tick: Document protections for tick related data

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Message-Id: <20201117132006.197713794@linutronix.de>
Date:   Tue, 17 Nov 2020 14:19:44 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     LKML <linux-kernel@...r.kernel.org>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Yunfeng Ye <yeyunfeng@...wei.com>
Subject: [patch 2/7] tick: Document protections for tick related data

The protection rules for tick_next_period and last_jiffies_update are blury
at best. Clarify this.

Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
---
 kernel/time/tick-common.c |    4 +++-
 kernel/time/tick-sched.c  |    4 +++-
 2 files changed, 6 insertions(+), 2 deletions(-)

--- a/kernel/time/tick-common.c
+++ b/kernel/time/tick-common.c
@@ -27,7 +27,9 @@
  */
 DEFINE_PER_CPU(struct tick_device, tick_cpu_device);
 /*
- * Tick next event: keeps track of the tick time
+ * Tick next event: keeps track of the tick time. It's updated by the
+ * CPU which handles the tick and protected by jiffies_lock. There is
+ * no requirement to write hold the jiffies seqcount for it.
  */
 ktime_t tick_next_period;
 ktime_t tick_period;
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -44,7 +44,9 @@ struct tick_sched *tick_get_tick_sched(i
 
 #if defined(CONFIG_NO_HZ_COMMON) || defined(CONFIG_HIGH_RES_TIMERS)
 /*
- * The time, when the last jiffy update happened. Protected by jiffies_lock.
+ * The time, when the last jiffy update happened. Write access must hold
+ * jiffies_lock and jiffies_seq. tick_nohz_next_event() needs to get a
+ * consistent view of jiffies and last_jiffies_update.
  */
 static ktime_t last_jiffies_update;