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);
> }
>
>
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