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vtime: Fix racy cputime delta update

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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 <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Paul E. McKenney <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Li Zhong <[email protected]>
Cc: Mike Galbraith <[email protected]>
Cc: Kevin Hilman <[email protected]>
This commit is contained in:
Frederic Weisbecker 2013-07-13 17:10:18 +02:00
commit 54461562c9
1 changed files with 2 additions and 1 deletions
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3
kernel/sched/cputime.c
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@ -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);