Re: [PATCH 1/2] locking/mutex: remove rcu_read_lock/unlock as we already disabled preemption

[Xu, Yanfei]

On 9/27/21 8:46 AM, Waiman Long wrote:
> [Please note: This e-mail is from an EXTERNAL e-mail address]
>
> On 9/26/21 3:16 PM, Waiman Long wrote:
> > On 9/26/21 6:16 AM, Yanfei Xu wrote:
> > > preempt_disable/enable() is equal to RCU read-side crital section,
> > > and the mutex lock slowpath disable the preemption throughout the
> > > entire slowpath. Let's remove the rcu_read_lock/unlock for saving
> > > some cycles in hot codes.
> >
> > The description is wrong. Preemption is disabled only in the
> > optimistic spinning code which is not the complete slowpath. Even
> > though it may sound reasonable that disable preemption is likely to
> > prevent reaching quiescent state, but I am not totally sure that will
> > always be the case as there are different RCU favors.
>
> It does look like that disable preemption can serve as a substitute for
> rcu_read_lock(). However, I will suggest that you also insert a comment
> saying so and the task structure won't go away during the spinning period.

Will send v2, thanks.

Yanfei

> Cheers,
> Longman