Re: [PATCH] sched/core: avoid spurious spinlock recursion splats

[Mark Rutland]

On Tue, Feb 06, 2018 at 11:03:07AM +0100, Peter Zijlstra wrote:
> On Mon, Feb 05, 2018 at 03:51:18PM +0000, Mark Rutland wrote:
> > However, this happens *after* prev->on_cpu is cleared, which allows prev
> > to be scheduled on another CPU. If prev then attempts to acquire the
> > same rq lock, before the updated rq->lock.owner is made visible, it will
> > see itself as the owner.
> 
> Cute.
> 
> > diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> > index b19552a212de..4f0d2e3701c3 100644
> > --- a/kernel/sched/sched.h
> > +++ b/kernel/sched/sched.h
> > @@ -1342,6 +1342,10 @@ static inline void prepare_lock_switch(struct rq *rq, struct task_struct *next)
> >  
> >  static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
> >  {
> > +#ifdef CONFIG_DEBUG_SPINLOCK
> > +	/* this is a valid case when another task releases the spinlock */
> > +	rq->lock.owner = current;
> > +#endif
> >  #ifdef CONFIG_SMP
> >  	/*
> >  	 * After ->on_cpu is cleared, the task can be moved to a different CPU.
> > @@ -1355,10 +1359,6 @@ static inline void finish_lock_switch(struct rq *rq, struct task_struct *prev)
> >  	 */
> >  	smp_store_release(&prev->on_cpu, 0);
> >  #endif
> > -#ifdef CONFIG_DEBUG_SPINLOCK
> > -	/* this is a valid case when another task releases the spinlock */
> > -	rq->lock.owner = current;
> > -#endif
> >  	/*
> >  	 * If we are tracking spinlock dependencies then we have to
> >  	 * fix up the runqueue lock - which gets 'carried over' from
> 
> Right, so patch:
> 
>   31cb1bc0dc94 ("sched/core: Rework and clarify prepare_lock_switch()")
> 
> munched all that code and the above no longer fits.

Ah, sorry. I based this on tip/sched-urgent-for-linus as of yesterday,
and I guess that was out-of-date.

> Does the below change also work for you? (tip/master)

So far it seems to in testing, and looks obviously correct to me.

If you're going to drop that on a branch, feel free to add my ack!

Thanks,
Mark.

> 
> ---
>  kernel/sched/core.c | 27 ++++++++++++++++-----------
>  1 file changed, 16 insertions(+), 11 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index ee420d78e674..abfd10692022 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2600,19 +2600,31 @@ static inline void finish_task(struct task_struct *prev)
>  #endif
>  }
>  
> -static inline void finish_lock_switch(struct rq *rq)
> +static inline void
> +prepare_lock_switch(struct rq *rq, struct task_struct *next, struct rq_flags *rf)
>  {
> +	/*
> +	 * Since the runqueue lock will be released by the next
> +	 * task (which is an invalid locking op but in the case
> +	 * of the scheduler it's an obvious special-case), so we
> +	 * do an early lockdep release here:
> +	 */
> +	rq_unpin_lock(rq, rf);
> +	spin_release(&rq->lock.dep_map, 1, _THIS_IP_);
>  #ifdef CONFIG_DEBUG_SPINLOCK
>  	/* this is a valid case when another task releases the spinlock */
> -	rq->lock.owner = current;
> +	rq->lock.owner = next;
>  #endif
> +}
> +
> +static inline void finish_lock_switch(struct rq *rq)
> +{
>  	/*
>  	 * If we are tracking spinlock dependencies then we have to
>  	 * fix up the runqueue lock - which gets 'carried over' from
>  	 * prev into current:
>  	 */
>  	spin_acquire(&rq->lock.dep_map, 0, 0, _THIS_IP_);
> -
>  	raw_spin_unlock_irq(&rq->lock);
>  }
>  
> @@ -2843,14 +2855,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
>  
>  	rq->clock_update_flags &= ~(RQCF_ACT_SKIP|RQCF_REQ_SKIP);
>  
> -	/*
> -	 * Since the runqueue lock will be released by the next
> -	 * task (which is an invalid locking op but in the case
> -	 * of the scheduler it's an obvious special-case), so we
> -	 * do an early lockdep release here:
> -	 */
> -	rq_unpin_lock(rq, rf);
> -	spin_release(&rq->lock.dep_map, 1, _THIS_IP_);
> +	prepare_lock_switch(rq, next, rf);
>  
>  	/* Here we just switch the register state and the stack. */
>  	switch_to(prev, next, prev);