Re: Re: [PATCH linux-next v2] sched/core: Add WARN() to check overflow in migrate_disable()

[Peter Zijlstra <peterz@infradead.org>]

On Fri, Jul 05, 2024 at 05:41:32AM +0000, Peilin He wrote:
> > How about we do something like this?
> >
> > ---
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 7a5ea8fc8abb..06a559532ed6 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -2237,6 +2237,12 @@ void migrate_disable(void)
> >  
> >      if (p->migration_disabled) {
> >          p->migration_disabled++;
> > +#ifdef CONFIG_DEBUG_PREEMPT
> > +        /*
> > +         * Warn about overflow half-way through the range.
> > +         */
> > +        WARN_ON_ONCE((s16)p->migration_disabled < 0);
> > +#endif
> >          return;
> >      }
> 
> Agreed, converting p->migration_disabled to a signed number 
> will detect overflow occurrences earlier than our current method.
> 
> > @@ -2254,14 +2260,20 @@ void migrate_enable(void)
> >          .flags     = SCA_MIGRATE_ENABLE,
> >      };
> >  
> > +#ifdef CONFIG_DEBUG_PREEMPT
> > +    /*
> > +     * Check both overflow from migrate_disable() and superfluous
> > +     * migrate_enable().
> > +     */
> > +    if (WARN_ON_ONCE((s16)p->migration_disabled <= 0))
> > +        return;
> > +#endif
> > +
> >      if (p->migration_disabled > 1) {
> >          p->migration_disabled--;
> >          return;
> >      }
> >  
> > -    if (WARN_ON_ONCE(!p->migration_disabled))
> > -        return;
> > -
> >      /*
> >       * Ensure stop_task runs either before or after this, and that
> >       * __set_cpus_allowed_ptr(SCA_MIGRATE_ENABLE) doesn't schedule().
> 
> Agreed, similarly checking for overflow in p->migration_disabled 
> within migrate_enable is fine, but placing WARN_ON_ONCE inside 
> the CONFIG_DEBUG_PREEMPT macro may cause return logic deviations. 
> It is recommended to support WARN_ON_ONCE as a standard feature.

It is debug stuff, it should only be needed for debug builds. Same as
the preemption things, they also only check for overflow on debug
builds.