Re: [PATCH] kthread: Fix race condition between kthread_parkme() and kthread_unpark()

[Andrew Morton <akpm@linux-foundation.org>]

(cc tglx & tj)

On Tue, 30 May 2017 15:37:23 -0700 Junaid Shahid <junaids@google.com> wrote:

> (Resending)
> 
> On Friday, April 28, 2017 07:32:36 PM Junaid Shahid wrote:
> > In general, if kthread_unpark() and kthread_parkme() execute together,
> > the kthread is supposed to be in an unparked state. This is because
> > kthread_unpark() either wakes up the thread if it already got parked,
> > or it cancels a prior kthread_park() call and hence renders the
> > kthread_parkme() moot.
> > 
> > However, if kthread_unpark() happens to execute between the time that
> > kthread_parkme() checks the KTHREAD_SHOULD_STOP flag and sets the
> > KTHREAD_IS_PARKED flag, then kthread_unpark() will not wake up the
> > thread and it will remain in a parked state.
> > 
> > This is fixed by making the checking of KTHREAD_SHOULD_STOP and the
> > setting of KTHREAD_IS_PARKED atomic via a cmpxchg inside kthread_parkme.
> > 
> > Signed-off-by: Junaid Shahid <junaids@google.com>
> > ---
> >  kernel/kthread.c | 16 ++++++++++++----
> >  1 file changed, 12 insertions(+), 4 deletions(-)
> > 
> > diff --git a/kernel/kthread.c b/kernel/kthread.c
> > index 26db528c1d88..651f03baf62f 100644
> > --- a/kernel/kthread.c
> > +++ b/kernel/kthread.c
> > @@ -169,12 +169,20 @@ void *kthread_probe_data(struct task_struct *task)
> >  
> >  static void __kthread_parkme(struct kthread *self)
> >  {
> > +	ulong flags;
> > +
> >  	__set_current_state(TASK_PARKED);
> > -	while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) {
> > -		if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags))
> > -			complete(&self->parked);
> > -		schedule();
> > +	flags = self->flags;
> > +
> > +	while (test_bit(KTHREAD_SHOULD_PARK, &flags)) {
> > +		if (cmpxchg(&self->flags, flags,
> > +			    flags | (1 << KTHREAD_IS_PARKED)) == flags) {
> > +			if (!test_bit(KTHREAD_IS_PARKED, &flags))
> > +				complete(&self->parked);
> > +			schedule();
> > +		}
> >  		__set_current_state(TASK_PARKED);
> > +		flags = self->flags;
> >  	}
> >  	clear_bit(KTHREAD_IS_PARKED, &self->flags);
> >  	__set_current_state(TASK_RUNNING);
> >