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softlockup: ensure the task has been switched out once 

When we check if a task has been switched out since the last scan, we might
have a race condition on the following scenario:

- the task is freshly created and scheduled

- it puts its state to TASK_UNINTERRUPTIBLE and is not yet switched out

- check_hung_task() scans this task and will report a false positive because
  t->nvcsw + t->nivcsw == t->last_switch_count == 0

Add a check for such cases.

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Acked-by: Mandeep Singh Baines <msb@google.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
This commit is contained in:
Frederic Weisbecker 2009-02-10 16:52:37 +01:00 committed by Ingo Molnar
parent 17406b82d6
commit cf2592f59c
1 changed files with 7 additions and 1 deletions
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8
kernel/hung_task.c
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@ -72,7 +72,13 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
{
unsigned long switch_count = t->nvcsw + t->nivcsw;
if (t->flags & PF_FROZEN)
/*
* Ensure the task is not frozen.
* Also, when a freshly created task is scheduled once, changes
* its state to TASK_UNINTERRUPTIBLE without having ever been
* switched out once, it musn't be checked.
*/
if (unlikely(t->flags & PF_FROZEN || !switch_count))
return;
if (switch_count != t->last_switch_count) {