#include <linux/atomic.h>#include <linux/rwsem.h>#include <linux/percpu.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 *sem,			const char *name, struct lock_class_key *rwsem_key){	sem->read_count = alloc_percpu(int);	if (unlikely(!sem->read_count))		return -ENOMEM; 	/* ->rw_sem represents the whole percpu_rw_semaphore for lockdep */	rcu_sync_init(&sem->rss, RCU_SCHED_SYNC);	__init_rwsem(&sem->rw_sem, name, rwsem_key);	rcuwait_init(&sem->writer);	sem->readers_block = 0;	return 0;}EXPORT_SYMBOL_GPL(__percpu_init_rwsem); void percpu_free_rwsem(struct percpu_rw_semaphore *sem){	/*	 * XXX: temporary kludge. The error path in alloc_super()	 * assumes that percpu_free_rwsem() is safe after kzalloc().	 */	if (!sem->read_count)		return; 	rcu_sync_dtor(&sem->rss);	free_percpu(sem->read_count);	sem->read_count = NULL; /* catch use after free bugs */}EXPORT_SYMBOL_GPL(percpu_free_rwsem); int __percpu_down_read(struct percpu_rw_semaphore *sem, int try){	/*	 * Due to having preemption disabled the decrement happens on	 * the same CPU as the increment, avoiding the	 * increment-on-one-CPU-and-decrement-on-another problem.	 *	 * If the reader misses the writer's assignment of readers_block, then	 * the writer is guaranteed to see the reader's increment.	 *	 * Conversely, any readers that increment their sem->read_count after	 * the writer looks are guaranteed to see the readers_block value,	 * which in turn means that they are guaranteed to immediately	 * decrement their sem->read_count, so that it doesn't matter that the	 * writer missed them.	 */ 	smp_mb(); /* A matches D */ 	/*	 * If !readers_block the critical section starts here, matched by the	 * release in percpu_up_write().	 */	if (likely(!smp_load_acquire(&sem->readers_block)))		return 1; 	/*	 * Per the above comment; we still have preemption disabled and	 * will thus decrement on the same CPU as we incremented.	 */	__percpu_up_read(sem); 	if (try)		return 0; 	/*	 * We either call schedule() in the wait, or we'll fall through	 * and reschedule on the preempt_enable() in percpu_down_read().	 */	preempt_enable_no_resched(); 	/*	 * Avoid lockdep for the down/up_read() we already have them.	 */	__down_read(&sem->rw_sem);	this_cpu_inc(*sem->read_count);	__up_read(&sem->rw_sem); 	preempt_disable();	return 1;}EXPORT_SYMBOL_GPL(__percpu_down_read); void __percpu_up_read(struct percpu_rw_semaphore *sem){	smp_mb(); /* B matches C */	/*	 * In other words, if they see our decrement (presumably to aggregate	 * zero, as that is the only time it matters) they will also see our	 * critical section.	 */	__this_cpu_dec(*sem->read_count); 	/* Prod writer to recheck readers_active */	rcuwait_wake_up(&sem->writer);}EXPORT_SYMBOL_GPL(__percpu_up_read); #define per_cpu_sum(var)						\({									\	typeof(var) __sum = 0;						\	int cpu;							\	compiletime_assert_atomic_type(__sum);				\	for_each_possible_cpu(cpu)					\		__sum += per_cpu(var, cpu);				\	__sum;								\}) /* * Return true if the modular sum of the sem->read_count per-CPU variable is * zero.  If this sum is zero, then it is stable due to the fact that if any * newly arriving readers increment a given counter, they will immediately * decrement that same counter. */static bool readers_active_check(struct percpu_rw_semaphore *sem){	if (per_cpu_sum(*sem->read_count) != 0)		return false; 	/*	 * If we observed the decrement; ensure we see the entire critical	 * section.	 */ 	smp_mb(); /* C matches B */ 	return true;} void percpu_down_write(struct percpu_rw_semaphore *sem){	/* Notify readers to take the slow path. */	rcu_sync_enter(&sem->rss); 	down_write(&sem->rw_sem); 	/*	 * Notify new readers to block; up until now, and thus throughout the	 * longish rcu_sync_enter() above, new readers could still come in.	 */	WRITE_ONCE(sem->readers_block, 1); 	smp_mb(); /* D matches A */ 	/*	 * If they don't see our writer of readers_block, then we are	 * guaranteed to see their sem->read_count increment, and therefore	 * will wait for them.	 */ 	/* Wait for all now active readers to complete. */	rcuwait_wait_event(&sem->writer, readers_active_check(sem));}EXPORT_SYMBOL_GPL(percpu_down_write); void percpu_up_write(struct percpu_rw_semaphore *sem){	/*	 * Signal the writer is done, no fast path yet.	 *	 * One reason that we cannot just immediately flip to readers_fast is	 * that new readers might fail to see the results of this writer's	 * critical section.	 *	 * Therefore we force it through the slow path which guarantees an	 * acquire and thereby guarantees the critical section's consistency.	 */	smp_store_release(&sem->readers_block, 0); 	/*	 * Release the write lock, this will allow readers back in the game.	 */	up_write(&sem->rw_sem); 	/*	 * Once this completes (at least one RCU-sched grace period hence) the	 * reader fast path will be available again. Safe to use outside the	 * exclusive write lock because its counting.	 */	rcu_sync_exit(&sem->rss);}EXPORT_SYMBOL_GPL(percpu_up_write);