Re: [PATCH 04/19] sched: Prepare for Core-wide rq->lock

[Josh Don]

On Wed, Apr 28, 2021 at 2:13 AM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> On Tue, Apr 27, 2021 at 04:30:02PM -0700, Josh Don wrote:
>
> > Also, did you mean to have a preempt_enable_no_resched() rather than
> > prempt_enable() in raw_spin_rq_trylock?
>
> No, trylock really needs to be preempt_enable(), because it can have
> failed, at which point it will not have incremented the preemption count
> and our decrement can hit 0, at which point we really should reschedule.

Ah yes, makes sense. Did you want to use preempt_enable_no_resched()
at all then? No chance of preempt_count() being 1 at the point of
enable, as you comment below.

> > I went over the rq_lockp stuff again after Don's reported lockup. Most
> > uses are safe due to already holding an rq lock. However,
> > double_rq_unlock() is prone to race:
> >
> > double_rq_unlock(rq1, rq2):
> > /* Initial state: core sched enabled, and rq1 and rq2 are smt
> > siblings. So, double_rq_lock(rq1, rq2) only took a single rq lock */
> > raw_spin_rq_unlock(rq1);
> > /* now not holding any rq lock */
> > /* sched core disabled. Now __rq_lockp(rq1) != __rq_lockp(rq2), so we
> > falsely unlock rq2 */
> > if (__rq_lockp(rq1) != __rq_lockp(rq2))
> >         raw_spin_rq_unlock(rq2);
> > else
> >         __release(rq2->lock);
> >
> > Instead we can cache __rq_lockp(rq1) and __rq_lockp(rq2) before
> > releasing the lock, in order to prevent this. FWIW I think it is
> > likely that Don is seeing a different issue.
>
> Ah, indeed so.. rq_lockp() could do with an assertion, not sure how to
> sanely do that. Anyway, double_rq_unlock() is simple enough to fix, we
> can simply flip the unlock()s.
>
> ( I'm suffering a cold and am really quite slow atm )

No worries, hope it's a mild one.

> How's this then?

Looks good to me (other than the synchronize_sched()->synchronize_rcu()).

For these locking patches,
Reviewed-by: Josh Don <joshdon@xxxxxxxxxx>

I'll see if I can repro  that lockup.

> ---
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f732642e3e09..3a534c0c1c46 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -290,6 +290,10 @@ static void sched_core_assert_empty(void)
>  static void __sched_core_enable(void)
>  {
>         static_branch_enable(&__sched_core_enabled);
> +       /*
> +        * Ensure raw_spin_rq_*lock*() have completed before flipping.
> +        */
> +       synchronize_sched();
>         __sched_core_flip(true);
>         sched_core_assert_empty();
>  }
> @@ -449,16 +453,23 @@ void raw_spin_rq_lock_nested(struct rq *rq, int subclass)
>  {
>         raw_spinlock_t *lock;
>
> +       /* Matches synchronize_sched() in __sched_core_enabled() */
> +       preempt_disable();
>         if (sched_core_disabled()) {
>                 raw_spin_lock_nested(&rq->__lock, subclass);
> +               /* preempt-count *MUST* be > 1 */
> +               preempt_enable_no_resched();
>                 return;
>         }
>
>         for (;;) {
>                 lock = __rq_lockp(rq);
>                 raw_spin_lock_nested(lock, subclass);
> -               if (likely(lock == __rq_lockp(rq)))
> +               if (likely(lock == __rq_lockp(rq))) {
> +                       /* preempt-count *MUST* be > 1 */
> +                       preempt_enable_no_resched();
>                         return;
> +               }
>                 raw_spin_unlock(lock);
>         }
>  }
> @@ -468,14 +479,21 @@ bool raw_spin_rq_trylock(struct rq *rq)
>         raw_spinlock_t *lock;
>         bool ret;
>
> -       if (sched_core_disabled())
> -               return raw_spin_trylock(&rq->__lock);
> +       /* Matches synchronize_sched() in __sched_core_enabled() */
> +       preempt_disable();
> +       if (sched_core_disabled()) {
> +               ret = raw_spin_trylock(&rq->__lock);
> +               preempt_enable();
> +               return ret;
> +       }
>
>         for (;;) {
>                 lock = __rq_lockp(rq);
>                 ret = raw_spin_trylock(lock);
> -               if (!ret || (likely(lock == __rq_lockp(rq))))
> +               if (!ret || (likely(lock == __rq_lockp(rq)))) {
> +                       preempt_enable();
>                         return ret;
> +               }
>                 raw_spin_unlock(lock);
>         }
>  }
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 6a905fe19eef..c9a52231d58a 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2568,11 +2568,12 @@ static inline void double_rq_unlock(struct rq *rq1, struct rq *rq2)
>         __releases(rq1->lock)
>         __releases(rq2->lock)
>  {
> -       raw_spin_rq_unlock(rq1);
>         if (__rq_lockp(rq1) != __rq_lockp(rq2))
>                 raw_spin_rq_unlock(rq2);
>         else
>                 __release(rq2->lock);
> +
> +       raw_spin_rq_unlock(rq1);
>  }
>
>  extern void set_rq_online (struct rq *rq);