[tip: timers/core] tick: Document protections for tick related data

[tip-bot2 for Thomas Gleixner]

The following commit has been merged into the timers/core branch of tip:

Commit-ID:     c398960cd82b233886fbff163986f998b5a5c008
Gitweb:        https://git.kernel.org/tip/c398960cd82b233886fbff163986f998b5a5c008
Author:        Thomas Gleixner <tglx@xxxxxxxxxxxxx>
AuthorDate:    Tue, 17 Nov 2020 14:19:44 +01:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Thu, 19 Nov 2020 10:48:28 +01:00

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@xxxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20201117132006.197713794@xxxxxxxxxxxxx

---
 kernel/time/tick-common.c | 4 +++-
 kernel/time/tick-sched.c  | 4 +++-
 2 files changed, 6 insertions(+), 2 deletions(-)

diff --git a/kernel/time/tick-common.c b/kernel/time/tick-common.c
index 6c9c342..68504eb 100644
--- 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;
diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
index 81632cd..15360e6 100644
--- a/kernel/time/tick-sched.c
+++ b/kernel/time/tick-sched.c
@@ -44,7 +44,9 @@ struct tick_sched *tick_get_tick_sched(int cpu)
 
 #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;