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

Peter Zijlstra
2019-10-29T18:47:56Z

On Wed, Aug 07, 2019 at 11:56:58AM +0200, Oleg Nesterov wrote:

> and either way, with or without 2 queues, what do you think about the code
> below?

Sorry for being so tardy with this thread.. having once again picked up
the patch, I found your email.

> This way the new reader does wake_up() only in the very unlikely case when
> it races with the new writer which sets sem->block = 1 right after
> this_cpu_inc().

Ah, by waiting early, you avoid spurious wakeups when
__percpu_down_read() happens after a successful percpu_down_write().
Nice!

I've made these changes. Now let me go have a play with that second
waitqueue.

> -------------------------------------------------------------------------------
>
> static inline void percpu_down_read(struct percpu_rw_semaphore *sem)
> {
> might_sleep();
> rwsem_acquire_read(&sem->dep_map, 0, 0, _RET_IP_);
>
> preempt_disable();
>
> if (likely(rcu_sync_is_idle(&sem->rss)))
> __this_cpu_inc(*sem->read_count);
> else
> __percpu_down_read(sem, false);
>
> preempt_enable();
> }
>
> static inline void percpu_up_read(struct percpu_rw_semaphore *sem)
> {
> rwsem_release(&sem->dep_map, 1, _RET_IP_);
>
> preempt_disable();
>
> if (likely(rcu_sync_is_idle(&sem->rss)))
> __this_cpu_dec(*sem->read_count);
> else
> __percpu_up_read(sem);
>
> preempt_enable();
> }

I like that symmetry, but see below ...

> // both called and return with preemption disabled
>
> bool __percpu_down_read(struct percpu_rw_semaphore *sem, bool try)
> {
>
> if (atomic_read_acquire(&sem->block)) {
> again:
> preempt_enable();
> __wait_event(sem->waiters, !atomic_read_acquire(&sem->block));
> preempt_disable();
> }
>
> __this_cpu_inc(*sem->read_count);
>
> smp_mb();
>
> if (likely(!atomic_read_acquire(&sem->block)))
> return true;
>
> __percpu_up_read(sem);
>
> if (try)
> return false;
>
> goto again;
> }
>
> void __percpu_up_read(struct percpu_rw_semaphore *sem)
> {
> smp_mb();
>
> __this_cpu_dec(*sem->read_count);
>
preempt_enable();
> wake_up(&sem->waiters);
preempt_disable()

and this (sadly) means there's a bunch of back-to-back
preempt_disable()+preempt_enable() calls. Leaving out the
preempt_disable() here makes it ugly again :/

Admittedly, this is PREEMPT_RT only, but given that is >< close to
mainline we'd better get it right.

> }
>