Re: [PATCH] locking/percpu_rwsem: Rewrite to not use rwsem

Boqun Feng
2019-08-06T14:15:34Z

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On Mon, Aug 05, 2019 at 05:43:28PM +0200, Peter Zijlstra wrote:
> On Mon, Aug 05, 2019 at 10:58:13PM +0800, Boqun Feng wrote:
> > On Mon, Aug 05, 2019 at 10:43:18PM +0800, Boqun Feng wrote:
> > > On Mon, Aug 05, 2019 at 04:02:41PM +0200, Peter Zijlstra wrote:
> > > [...]
> > > >
> > > > static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
> > > > {
> > > > + rwsem_release(&sem->dep_map, 1, _RET_IP_);
> > > > +
> > > > preempt_disable();
> > > > /*
> > > > * Same as in percpu_down_read().
> > > > */
> > > > - if (likely(rcu_sync_is_idle(&sem->rss)))
> > > > + if (likely(rcu_sync_is_idle(&sem->rss))) {
> > > > __this_cpu_dec(*sem->read_count);
> > > > - else
> > > > - __percpu_up_read(sem); /* Unconditional memory barrier */
> > > > - preempt_enable();
> > > > + preempt_enable();
> > > > + return;
> > > > + }
> > > >
> > > > - rwsem_release(&sem->rw_sem.dep_map, 1, _RET_IP_);
> > >
> > > Missing a preempt_enable() here?
> > >
> >
> > Ah.. you modified the semantics of __percpu_up_read() to imply a
> > preempt_enable(), sorry for the noise...
>
> Yes indeed; I suppose I should've noted that in the Changlog. The reason
> is that waitqueues use spin_lock() which change into a sleepable lock on
> RT and thus cannot be used with preeption disabled. We also cannot
> (easily) switch to swait because we use both exclusive and !exclusive
> waits.

Thanks for the explanation. I was missing the point that the modfication
is mostly for RT, much clear now ;-)

Regards,
Boqun

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