Messages in this thread | | | Subject | Re: [dm-devel] [PATCH 2/3] Introduce percpu rw semaphores | From | Eric Dumazet <> | Date | Sun, 29 Jul 2012 20:36:02 +0200 |
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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() */ 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() */
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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