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

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]

 



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);
}


--
To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html


[Index of Archives]     [Linux Ext4 Filesystem]     [Union Filesystem]     [Filesystem Testing]     [Ceph Users]     [Ecryptfs]     [AutoFS]     [Kernel Newbies]     [Share Photos]     [Security]     [Netfilter]     [Bugtraq]     [Yosemite News]     [MIPS Linux]     [ARM Linux]     [Linux Security]     [Linux Cachefs]     [Reiser Filesystem]     [Linux RAID]     [Samba]     [Device Mapper]     [CEPH Development]
  Powered by Linux