Re: [PATCH] softlockup: ensure the task has been switched out once

[Ingo Molnar]

* Mandeep Singh Baines <msb@xxxxxxxxxx> wrote:

> Frederic Weisbecker (fweisbec@xxxxxxxxx) wrote:
> > 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@xxxxxxxxx>
> > ---
> >  kernel/hung_task.c |    8 +++++++-
> >  1 files changed, 7 insertions(+), 1 deletions(-)
> > 
> > diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> > index 0c924de..022a492 100644
> > --- a/kernel/hung_task.c
> > +++ b/kernel/hung_task.c
> > @@ -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) {
> > -- 
> > 1.6.1
> > 
> > 
> 
> 
> Looks good to me. Thanks for fixing this!
> 
> Acked-by: Mandeep Singh Baines <msb@xxxxxxxxxx>

Applied to tip:core/softlockup, thanks guys!

	Ingo
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