#include <linux/atomic.h>#include <linux/rwsem.h>#include <linux/percpu.h>#include <linux/wait.h>#include <linux/lockdep.h>#include <linux/percpu-rwsem.h>#include <linux/rcupdate.h>#include <linux/sched.h>#include <linux/errno.h> int __percpu_init_rwsem(struct percpu_rw_semaphore *brw,			const char *name, struct lock_class_key *rwsem_key){	brw->fast_read_ctr = alloc_percpu(int);	if (unlikely(!brw->fast_read_ctr))		return -ENOMEM; 	/* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */	__init_rwsem(&brw->rw_sem, name, rwsem_key);	rcu_sync_init(&brw->rss, RCU_SCHED_SYNC);	atomic_set(&brw->slow_read_ctr, 0);	init_waitqueue_head(&brw->write_waitq);	return 0;}EXPORT_SYMBOL_GPL(__percpu_init_rwsem); void percpu_free_rwsem(struct percpu_rw_semaphore *brw){	/*	 * XXX: temporary kludge. The error path in alloc_super()	 * assumes that percpu_free_rwsem() is safe after kzalloc().	 */	if (!brw->fast_read_ctr)		return; 	rcu_sync_dtor(&brw->rss);	free_percpu(brw->fast_read_ctr);	brw->fast_read_ctr = NULL; /* catch use after free bugs */} /* * This is the fast-path for down_read/up_read. If it succeeds we rely * on the barriers provided by rcu_sync_enter/exit; see the comments in * percpu_down_write() and percpu_up_write(). * * If this helper fails the callers rely on the normal rw_semaphore and * atomic_dec_and_test(), so in this case we have the necessary barriers. */static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val){	bool success; 	preempt_disable();	success = rcu_sync_is_idle(&brw->rss);	if (likely(success))		__this_cpu_add(*brw->fast_read_ctr, val);	preempt_enable(); 	return success;} /* * Like the normal down_read() this is not recursive, the writer can * come after the first percpu_down_read() and create the deadlock. * * Note: returns with lock_is_held(brw->rw_sem) == T for lockdep, * percpu_up_read() does rwsem_release(). This pairs with the usage * of ->rw_sem in percpu_down/up_write(). */void percpu_down_read(struct percpu_rw_semaphore *brw){	might_sleep();	rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_); 	if (likely(update_fast_ctr(brw, +1)))		return; 	/* Avoid rwsem_acquire_read() and rwsem_release() */	__down_read(&brw->rw_sem);	atomic_inc(&brw->slow_read_ctr);	__up_read(&brw->rw_sem);}EXPORT_SYMBOL_GPL(percpu_down_read); int percpu_down_read_trylock(struct percpu_rw_semaphore *brw){	if (unlikely(!update_fast_ctr(brw, +1))) {		if (!__down_read_trylock(&brw->rw_sem))			return 0;		atomic_inc(&brw->slow_read_ctr);		__up_read(&brw->rw_sem);	} 	rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 1, _RET_IP_);	return 1;} void percpu_up_read(struct percpu_rw_semaphore *brw){	rwsem_release(&brw->rw_sem.dep_map, 1, _RET_IP_); 	if (likely(update_fast_ctr(brw, -1)))		return; 	/* false-positive is possible but harmless */	if (atomic_dec_and_test(&brw->slow_read_ctr))		wake_up_all(&brw->write_waitq);}EXPORT_SYMBOL_GPL(percpu_up_read); static int clear_fast_ctr(struct percpu_rw_semaphore *brw){	unsigned int sum = 0;	int cpu; 	for_each_possible_cpu(cpu) {		sum += per_cpu(*brw->fast_read_ctr, cpu);		per_cpu(*brw->fast_read_ctr, cpu) = 0;	} 	return sum;} void percpu_down_write(struct percpu_rw_semaphore *brw){	/*	 * Make rcu_sync_is_idle() == F and thus disable the fast-path in	 * percpu_down_read() and percpu_up_read(), and wait for gp pass.	 *	 * The latter synchronises us with the preceding readers which used	 * the fast-past, so we can not miss the result of __this_cpu_add()	 * or anything else inside their criticial sections.	 */	rcu_sync_enter(&brw->rss); 	/* exclude other writers, and block the new readers completely */	down_write(&brw->rw_sem); 	/* nobody can use fast_read_ctr, move its sum into slow_read_ctr */	atomic_add(clear_fast_ctr(brw), &brw->slow_read_ctr); 	/* wait for all readers to complete their percpu_up_read() */	wait_event(brw->write_waitq, !atomic_read(&brw->slow_read_ctr));}EXPORT_SYMBOL_GPL(percpu_down_write); void percpu_up_write(struct percpu_rw_semaphore *brw){	/* release the lock, but the readers can't use the fast-path */	up_write(&brw->rw_sem);	/*	 * Enable the fast-path in percpu_down_read() and percpu_up_read()	 * but only after another gp pass; this adds the necessary barrier	 * to ensure the reader can't miss the changes done by us.	 */	rcu_sync_exit(&brw->rss);}EXPORT_SYMBOL_GPL(percpu_up_write);