#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);	atomic_set(&brw->write_ctr, 0);	atomic_set(&brw->slow_read_ctr, 0);	init_waitqueue_head(&brw->write_waitq);	return 0;} void percpu_free_rwsem(struct percpu_rw_semaphore *brw){	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, it only needs to ensure * there is no pending writer (atomic_read(write_ctr) == 0) and inc/dec the * fast per-cpu counter. The writer uses synchronize_sched_expedited() to * serialize with the preempt-disabled section below. * * The nontrivial part is that we should guarantee acquire/release semantics * in case when * *	R_W: down_write() comes after up_read(), the writer should see all *	     changes done by the reader * or *	W_R: down_read() comes after up_write(), the reader should see all *	     changes done by the writer * * 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. * * But if it succeeds we do not have any barriers, atomic_read(write_ctr) or * __this_cpu_add() below can be reordered with any LOAD/STORE done by the * reader inside the critical section. See the comments in down_write and * up_write below. */static bool update_fast_ctr(struct percpu_rw_semaphore *brw, unsigned int val){	bool success = false; 	preempt_disable();	if (likely(!atomic_read(&brw->write_ctr))) {		__this_cpu_add(*brw->fast_read_ctr, val);		success = true;	}	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();	if (likely(update_fast_ctr(brw, +1))) {		rwsem_acquire_read(&brw->rw_sem.dep_map, 0, 0, _RET_IP_);		return;	} 	down_read(&brw->rw_sem);	atomic_inc(&brw->slow_read_ctr);	/* avoid up_read()->rwsem_release() */	__up_read(&brw->rw_sem);} 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);} 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;} /* * A writer increments ->write_ctr to force the readers to switch to the * slow mode, note the atomic_read() check in update_fast_ctr(). * * After that the readers can only inc/dec the slow ->slow_read_ctr counter, * ->fast_read_ctr is stable. Once the writer moves its sum into the slow * counter it represents the number of active readers. * * Finally the writer takes ->rw_sem for writing and blocks the new readers, * then waits until the slow counter becomes zero. */void percpu_down_write(struct percpu_rw_semaphore *brw){	/* tell update_fast_ctr() there is a pending writer */	atomic_inc(&brw->write_ctr);	/*	 * 1. Ensures that write_ctr != 0 is visible to any down_read/up_read	 *    so that update_fast_ctr() can't succeed.	 *	 * 2. Ensures we see the result of every previous this_cpu_add() in	 *    update_fast_ctr().	 *	 * 3. Ensures that if any reader has exited its critical section via	 *    fast-path, it executes a full memory barrier before we return.	 *    See R_W case in the comment above update_fast_ctr().	 */	synchronize_sched_expedited(); 	/* 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));} 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);	/*	 * Insert the barrier before the next fast-path in down_read,	 * see W_R case in the comment above update_fast_ctr().	 */	synchronize_sched_expedited();	/* the last writer unblocks update_fast_ctr() */	atomic_dec(&brw->write_ctr);}