[PATCH 03/21] vtime: Update a few comments

[Frederic Weisbecker]

Update a stale comment from the old vtime era and document some
locking that might be non obvious.

Signed-off-by: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Ingo Molnar <mingo@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Li Zhong <zhong@xxxxxxxxxxxxxxxxxx>
Cc: Mike Galbraith <efault@xxxxxx>
Cc: Kevin Hilman <khilman@xxxxxxxxxx>
---
 include/linux/context_tracking.h |   10 ++++------
 kernel/sched/cputime.c           |    7 +++++++
 2 files changed, 11 insertions(+), 6 deletions(-)

diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index 5984f25..d883ff0 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -72,8 +72,9 @@ extern void guest_exit(void);
 static inline void guest_enter(void)
 {
 	/*
-	 * This is running in ioctl context so we can avoid
-	 * the call to vtime_account() with its unnecessary idle check.
+	 * This is running in ioctl context so its safe
+	 * to assume that it's the stime pending cputime
+	 * to flush.
 	 */
 	vtime_account_system(current);
 	current->flags |= PF_VCPU;
@@ -81,10 +82,7 @@ static inline void guest_enter(void)
 
 static inline void guest_exit(void)
 {
-	/*
-	 * This is running in ioctl context so we can avoid
-	 * the call to vtime_account() with its unnecessary idle check.
-	 */
+	/* Flush the guest cputime we spent on the guest */
 	vtime_account_system(current);
 	current->flags &= ~PF_VCPU;
 }
diff --git a/kernel/sched/cputime.c b/kernel/sched/cputime.c
index a7959e0..223a35e 100644
--- a/kernel/sched/cputime.c
+++ b/kernel/sched/cputime.c
@@ -712,6 +712,13 @@ void vtime_user_enter(struct task_struct *tsk)
 
 void vtime_guest_enter(struct task_struct *tsk)
 {
+	/*
+	 * The flags must be updated under the lock with
+	 * the vtime_snap flush and update.
+	 * That enforces a right ordering and update sequence
+	 * synchronization against the reader (task_gtime())
+	 * that can thus safely catch up with a tickless delta.
+	 */
 	write_seqlock(&tsk->vtime_seqlock);
 	__vtime_account_system(tsk);
 	current->flags |= PF_VCPU;
-- 
1.7.5.4

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