Re: [PATCH 2/2] locking/rwsem: Use rcu_read_lock_sched to simplify codes

[Xu, Yanfei]

On 9/27/21 3:22 AM, Waiman Long wrote:
> [Please note: This e-mail is from an EXTERNAL e-mail address]
>
> On 9/26/21 6:16 AM, Yanfei Xu wrote:
> > Use rcu_read_lock_sched to simplify the codes, and it also saves
> > some cycles of handling rcu nesting counter.
> >
> > Signed-off-by: Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx>
> > ---
> >   kernel/locking/rwsem.c | 6 ++----
> >   1 file changed, 2 insertions(+), 4 deletions(-)
> >
> > diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> > index 000e8d5a2884..7afadfe02286 100644
> > --- a/kernel/locking/rwsem.c
> > +++ b/kernel/locking/rwsem.c
> > @@ -616,8 +616,7 @@ static inline bool rwsem_can_spin_on_owner(struct 
> > rw_semaphore *sem)
> >               return false;
> >       }
> >
> > -     preempt_disable();
> > -     rcu_read_lock();
> > +     rcu_read_lock_sched();
> >       owner = rwsem_owner_flags(sem, &flags);
> >       /*
> >        * Don't check the read-owner as the entry may be stale.
> > @@ -625,8 +624,7 @@ static inline bool rwsem_can_spin_on_owner(struct 
> > rw_semaphore *sem)
> >       if ((flags & RWSEM_NONSPINNABLE) ||
> >           (owner && !(flags & RWSEM_READER_OWNED) && 
> > !owner_on_cpu(owner)))
> >               ret = false;
> > -     rcu_read_unlock();
> > -     preempt_enable();
> > +     rcu_read_unlock_sched();
> >
> >       lockevent_cond_inc(rwsem_opt_fail, !ret);
> >       return ret;
>
> I don't think there is any performance gain with this change. I would
> prefer the original code that is more readable as some people may not
> know rcu_read_lock_sched() will disable preemption if they don't look
> into it.

OK, thanks for comments.

Yanfei

> Cheers,
> Longman