vtime: Fix racy cputime delta update

get_vtime_delta() must be called under the task vtime_seqlock
with the code that does the cputime accounting flush.

Otherwise the cputime reader can be fooled and run into
a race where it sees the snapshot update but misses the
cputime flush. As a result it can report a cputime that is
way too short.

Fix vtime_account_user() that wasn't complying to that rule.

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>

kernel/sched/cputime.c

@@ -683,9 +683,10 @@ void vtime_account_irq_exit(struct task_struct *tsk)
 
 void vtime_account_user(struct task_struct *tsk)
 {
-	cputime_t delta_cpu = get_vtime_delta(tsk);
+	cputime_t delta_cpu;
 
 	write_seqlock(&tsk->vtime_seqlock);
+	delta_cpu = get_vtime_delta(tsk);
 	tsk->vtime_snap_whence = VTIME_SYS;
 	account_user_time(tsk, delta_cpu, cputime_to_scaled(delta_cpu));
 	write_sequnlock(&tsk->vtime_seqlock);