vtime: Update a few comments

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

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Li Zhong <zhong@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Kevin Hilman <khilman@linaro.org>

2 files changed, 11 insertions, 6 deletions

diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index 5984f2556d13..d883ff0dd8fc 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 a7959e05a9d5..223a35efa0a6 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;