Re: [dm-devel] [PATCH 2/3] Introduce percpu rw semaphores

[Mikulas Patocka <mpatocka redhat com>]

On Sun, 29 Jul 2012, Eric Dumazet wrote:

> On Sun, 2012-07-29 at 12:10 +0200, Eric Dumazet wrote:
> 
> > You can probably design something needing no more than 4 bytes per cpu,
> > and this thing could use non locked operations as bonus.
> > 
> > like the following ...
> 
> Coming back from my bike ride, here is a more polished version with
> proper synchronization/ barriers.
> 
> struct percpu_rw_semaphore {
> 	/* percpu_sem_down_read() use the following in fast path */
> 	unsigned int __percpu *active_counters;
> 
> 	unsigned int __percpu *counters;
> 	struct rw_semaphore	sem; /* used in slow path and by writers */
> };
> 
> static inline int percpu_sem_init(struct percpu_rw_semaphore *p)
> {
> 	p->counters = alloc_percpu(unsigned int);
> 	if (!p->counters)
> 		return -ENOMEM;
> 	init_rwsem(&p->sem);
> 	rcu_assign_pointer(p->active_counters, p->counters);
> 	return 0;
> }
> 
> 
> static inline bool percpu_sem_down_read(struct percpu_rw_semaphore *p)
> {
> 	unsigned int __percpu *counters;
> 
> 	rcu_read_lock();
> 	counters = rcu_dereference(p->active_counters);
> 	if (counters) {
> 		this_cpu_inc(*counters);
> 		smp_wmb(); /* paired with smp_rmb() in percpu_count() */

Why is this barrier needed? RCU works as a barrier doesn't it?
RCU is unlocked when the cpu passes a quiescent state, and I suppose that 
entering the quiescent state works as a barrier. Or doesn't it?

> 		rcu_read_unlock();
> 		return true;
> 	}
> 	rcu_read_unlock();
> 	down_read(&p->sem);
> 	return false;
> }
> 
> static inline void percpu_sem_up_read(struct percpu_rw_semaphore *p, bool fastpath)
> {
> 	if (fastpath)
> 		this_cpu_dec(*p->counters);
> 	else
> 		up_read(&p->sem);
> }
> 
> static inline unsigned int percpu_count(unsigned int __percpu *counters)
> {
> 	unsigned int total = 0;
> 	int cpu;
> 
> 	for_each_possible_cpu(cpu)
> 		total += *per_cpu_ptr(counters, cpu);
> 
> 	return total;
> }
> 
> static inline void percpu_sem_down_write(struct percpu_rw_semaphore *p)
> {
> 	down_write(&p->sem);
> 	p->active_counters = NULL;
> 	synchronize_rcu();
> 	smp_rmb(); /* paired with smp_wmb() in percpu_sem_down_read() */

Why barrier here? Synchronize_rcu() doesn't work as a barrier?

Mikulas

> 	while (percpu_count(p->counters))
> 		schedule();
> }
> 
> static inline void percpu_sem_up_write(struct percpu_rw_semaphore *p)
> {
> 	rcu_assign_pointer(p->active_counters, p->counters);
> 	up_write(&p->sem);
> }
> 
>