Re: [PATCH v2] locking/mutex: remove rcu_read_lock/unlock as we already disabled preemption

[Xu, Yanfei]

On 10/12/21 9:15 PM, Peter Zijlstra wrote:
> [Please note: This e-mail is from an EXTERNAL e-mail address]
>
> On Fri, Oct 08, 2021 at 11:25:18AM +0800, Yanfei Xu wrote:
> > preempt_disable/enable() is equal to RCU read-side crital section,
> > and the mutex lock slowpath disabled the preemption for the optimistic
> > spinning code. Let's remove the rcu_read_lock/unlock for saving some
> > cycles in hot codes.
> >
> > Signed-off-by: Yanfei Xu <yanfei.xu@xxxxxxxxxxxxx>
> > ---
> > v1->v2: fix the incorrect comment in code and commit message.
> >          thanks for WaiMan's suggestion.
> >
> > BTW, sorry for this late v2 due to a long vocation.
> >
> >   kernel/locking/mutex.c | 10 +++++-----
> >   1 file changed, 5 insertions(+), 5 deletions(-)
> >
> > diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> > index 2fede72b6af5..2f654cfb10d9 100644
> > --- a/kernel/locking/mutex.c
> > +++ b/kernel/locking/mutex.c
> > @@ -351,13 +351,14 @@ bool mutex_spin_on_owner(struct mutex *lock, struct task_struct *owner,
> >   {
> >        bool ret = true;
>
>          lockdep_assert_preemption_disabled();

Agree.

> > -     rcu_read_lock();
> >        while (__mutex_owner(lock) == owner) {
> >                /*
> >                 * Ensure we emit the owner->on_cpu, dereference _after_
>
> And did you check the other code in locking/ for similar things?

I did a check, rwsem also has the similar things. Will do it for rwsem 
in v3.



Thanks,
Yanfei