#define pr_fmt(fmt) "%s: " fmt "\n", __func__ #include <linux/kernel.h>#include <linux/sched.h>#include <linux/wait.h>#include <linux/percpu-refcount.h> /* * Initially, a percpu refcount is just a set of percpu counters. Initially, we * don't try to detect the ref hitting 0 - which means that get/put can just * increment or decrement the local counter. Note that the counter on a * particular cpu can (and will) wrap - this is fine, when we go to shutdown the * percpu counters will all sum to the correct value * * (More precisely: because modular arithmetic is commutative the sum of all the * percpu_count vars will be equal to what it would have been if all the gets * and puts were done to a single integer, even if some of the percpu integers * overflow or underflow). * * The real trick to implementing percpu refcounts is shutdown. We can't detect * the ref hitting 0 on every put - this would require global synchronization * and defeat the whole purpose of using percpu refs. * * What we do is require the user to keep track of the initial refcount; we know * the ref can't hit 0 before the user drops the initial ref, so as long as we * convert to non percpu mode before the initial ref is dropped everything * works. * * Converting to non percpu mode is done with some RCUish stuff in * percpu_ref_kill. Additionally, we need a bias value so that the * atomic_long_t can't hit 0 before we've added up all the percpu refs. */ #define PERCPU_COUNT_BIAS	(1LU << (BITS_PER_LONG - 1)) static DEFINE_SPINLOCK(percpu_ref_switch_lock);static DECLARE_WAIT_QUEUE_HEAD(percpu_ref_switch_waitq); static unsigned long __percpu *percpu_count_ptr(struct percpu_ref *ref){	return (unsigned long __percpu *)		(ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC_DEAD);} /** * percpu_ref_init - initialize a percpu refcount * @ref: percpu_ref to initialize * @release: function which will be called when refcount hits 0 * @flags: PERCPU_REF_INIT_* flags * @gfp: allocation mask to use * * Initializes @ref.  If @flags is zero, @ref starts in percpu mode with a * refcount of 1; analagous to atomic_long_set(ref, 1).  See the * definitions of PERCPU_REF_INIT_* flags for flag behaviors. * * Note that @release must not sleep - it may potentially be called from RCU * callback context by percpu_ref_kill(). */int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release,		    unsigned int flags, gfp_t gfp){	size_t align = max_t(size_t, 1 << __PERCPU_REF_FLAG_BITS,			     __alignof__(unsigned long));	unsigned long start_count = 0; 	ref->percpu_count_ptr = (unsigned long)		__alloc_percpu_gfp(sizeof(unsigned long), align, gfp);	if (!ref->percpu_count_ptr)		return -ENOMEM; 	ref->force_atomic = flags & PERCPU_REF_INIT_ATOMIC; 	if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD))		ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;	else		start_count += PERCPU_COUNT_BIAS; 	if (flags & PERCPU_REF_INIT_DEAD)		ref->percpu_count_ptr |= __PERCPU_REF_DEAD;	else		start_count++; 	atomic_long_set(&ref->count, start_count); 	ref->release = release;	ref->confirm_switch = NULL;	return 0;}EXPORT_SYMBOL_GPL(percpu_ref_init); /** * percpu_ref_exit - undo percpu_ref_init() * @ref: percpu_ref to exit * * This function exits @ref.  The caller is responsible for ensuring that * @ref is no longer in active use.  The usual places to invoke this * function from are the @ref->release() callback or in init failure path * where percpu_ref_init() succeeded but other parts of the initialization * of the embedding object failed. */void percpu_ref_exit(struct percpu_ref *ref){	unsigned long __percpu *percpu_count = percpu_count_ptr(ref); 	if (percpu_count) {		/* non-NULL confirm_switch indicates switching in progress */		WARN_ON_ONCE(ref->confirm_switch);		free_percpu(percpu_count);		ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;	}}EXPORT_SYMBOL_GPL(percpu_ref_exit); static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu){	struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu); 	ref->confirm_switch(ref);	ref->confirm_switch = NULL;	wake_up_all(&percpu_ref_switch_waitq); 	/* drop ref from percpu_ref_switch_to_atomic() */	percpu_ref_put(ref);} static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu){	struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);	unsigned long __percpu *percpu_count = percpu_count_ptr(ref);	unsigned long count = 0;	int cpu; 	for_each_possible_cpu(cpu)		count += *per_cpu_ptr(percpu_count, cpu); 	pr_debug("global %ld percpu %ld",		 atomic_long_read(&ref->count), (long)count); 	/*	 * It's crucial that we sum the percpu counters _before_ adding the sum	 * to &ref->count; since gets could be happening on one cpu while puts	 * happen on another, adding a single cpu's count could cause	 * @ref->count to hit 0 before we've got a consistent value - but the	 * sum of all the counts will be consistent and correct.	 *	 * Subtracting the bias value then has to happen _after_ adding count to	 * &ref->count; we need the bias value to prevent &ref->count from	 * reaching 0 before we add the percpu counts. But doing it at the same	 * time is equivalent and saves us atomic operations:	 */	atomic_long_add((long)count - PERCPU_COUNT_BIAS, &ref->count); 	WARN_ONCE(atomic_long_read(&ref->count) <= 0,		  "percpu ref (%pf) <= 0 (%ld) after switching to atomic",		  ref->release, atomic_long_read(&ref->count)); 	/* @ref is viewed as dead on all CPUs, send out switch confirmation */	percpu_ref_call_confirm_rcu(rcu);} static void percpu_ref_noop_confirm_switch(struct percpu_ref *ref){} static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,					  percpu_ref_func_t *confirm_switch){	if (ref->percpu_count_ptr & __PERCPU_REF_ATOMIC) {		if (confirm_switch)			confirm_switch(ref);		return;	} 	/* switching from percpu to atomic */	ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC; 	/*	 * Non-NULL ->confirm_switch is used to indicate that switching is	 * in progress.  Use noop one if unspecified.	 */	ref->confirm_switch = confirm_switch ?: percpu_ref_noop_confirm_switch; 	percpu_ref_get(ref);	/* put after confirmation */	call_rcu_sched(&ref->rcu, percpu_ref_switch_to_atomic_rcu);} static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref){	unsigned long __percpu *percpu_count = percpu_count_ptr(ref);	int cpu; 	BUG_ON(!percpu_count); 	if (!(ref->percpu_count_ptr & __PERCPU_REF_ATOMIC))		return; 	atomic_long_add(PERCPU_COUNT_BIAS, &ref->count); 	/*	 * Restore per-cpu operation.  smp_store_release() is paired with	 * smp_read_barrier_depends() in __ref_is_percpu() and guarantees	 * that the zeroing is visible to all percpu accesses which can see	 * the following __PERCPU_REF_ATOMIC clearing.	 */	for_each_possible_cpu(cpu)		*per_cpu_ptr(percpu_count, cpu) = 0; 	smp_store_release(&ref->percpu_count_ptr,			  ref->percpu_count_ptr & ~__PERCPU_REF_ATOMIC);} static void __percpu_ref_switch_mode(struct percpu_ref *ref,				     percpu_ref_func_t *confirm_switch){	lockdep_assert_held(&percpu_ref_switch_lock); 	/*	 * If the previous ATOMIC switching hasn't finished yet, wait for	 * its completion.  If the caller ensures that ATOMIC switching	 * isn't in progress, this function can be called from any context.	 */	wait_event_lock_irq(percpu_ref_switch_waitq, !ref->confirm_switch,			    percpu_ref_switch_lock); 	if (ref->force_atomic || (ref->percpu_count_ptr & __PERCPU_REF_DEAD))		__percpu_ref_switch_to_atomic(ref, confirm_switch);	else		__percpu_ref_switch_to_percpu(ref);} /** * percpu_ref_switch_to_atomic - switch a percpu_ref to atomic mode * @ref: percpu_ref to switch to atomic mode * @confirm_switch: optional confirmation callback * * There's no reason to use this function for the usual reference counting. * Use percpu_ref_kill[_and_confirm](). * * Schedule switching of @ref to atomic mode.  All its percpu counts will * be collected to the main atomic counter.  On completion, when all CPUs * are guaraneed to be in atomic mode, @confirm_switch, which may not * block, is invoked.  This function may be invoked concurrently with all * the get/put operations and can safely be mixed with kill and reinit * operations.  Note that @ref will stay in atomic mode across kill/reinit * cycles until percpu_ref_switch_to_percpu() is called. * * This function may block if @ref is in the process of switching to atomic * mode.  If the caller ensures that @ref is not in the process of * switching to atomic mode, this function can be called from any context. */void percpu_ref_switch_to_atomic(struct percpu_ref *ref,				 percpu_ref_func_t *confirm_switch){	unsigned long flags; 	spin_lock_irqsave(&percpu_ref_switch_lock, flags); 	ref->force_atomic = true;	__percpu_ref_switch_mode(ref, confirm_switch); 	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);} /** * percpu_ref_switch_to_percpu - switch a percpu_ref to percpu mode * @ref: percpu_ref to switch to percpu mode * * There's no reason to use this function for the usual reference counting. * To re-use an expired ref, use percpu_ref_reinit(). * * Switch @ref to percpu mode.  This function may be invoked concurrently * with all the get/put operations and can safely be mixed with kill and * reinit operations.  This function reverses the sticky atomic state set * by PERCPU_REF_INIT_ATOMIC or percpu_ref_switch_to_atomic().  If @ref is * dying or dead, the actual switching takes place on the following * percpu_ref_reinit(). * * This function may block if @ref is in the process of switching to atomic * mode.  If the caller ensures that @ref is not in the process of * switching to atomic mode, this function can be called from any context. */void percpu_ref_switch_to_percpu(struct percpu_ref *ref){	unsigned long flags; 	spin_lock_irqsave(&percpu_ref_switch_lock, flags); 	ref->force_atomic = false;	__percpu_ref_switch_mode(ref, NULL); 	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);} /** * percpu_ref_kill_and_confirm - drop the initial ref and schedule confirmation * @ref: percpu_ref to kill * @confirm_kill: optional confirmation callback * * Equivalent to percpu_ref_kill() but also schedules kill confirmation if * @confirm_kill is not NULL.  @confirm_kill, which may not block, will be * called after @ref is seen as dead from all CPUs at which point all * further invocations of percpu_ref_tryget_live() will fail.  See * percpu_ref_tryget_live() for details. * * This function normally doesn't block and can be called from any context * but it may block if @confirm_kill is specified and @ref is in the * process of switching to atomic mode by percpu_ref_switch_to_atomic(). */void percpu_ref_kill_and_confirm(struct percpu_ref *ref,				 percpu_ref_func_t *confirm_kill){	unsigned long flags; 	spin_lock_irqsave(&percpu_ref_switch_lock, flags); 	WARN_ONCE(ref->percpu_count_ptr & __PERCPU_REF_DEAD,		  "%s called more than once on %pf!", __func__, ref->release); 	ref->percpu_count_ptr |= __PERCPU_REF_DEAD;	__percpu_ref_switch_mode(ref, confirm_kill);	percpu_ref_put(ref); 	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);}EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm); /** * percpu_ref_reinit - re-initialize a percpu refcount * @ref: perpcu_ref to re-initialize * * Re-initialize @ref so that it's in the same state as when it finished * percpu_ref_init() ignoring %PERCPU_REF_INIT_DEAD.  @ref must have been * initialized successfully and reached 0 but not exited. * * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while * this function is in progress. */void percpu_ref_reinit(struct percpu_ref *ref){	unsigned long flags; 	spin_lock_irqsave(&percpu_ref_switch_lock, flags); 	WARN_ON_ONCE(!percpu_ref_is_zero(ref)); 	ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;	percpu_ref_get(ref);	__percpu_ref_switch_mode(ref, NULL); 	spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);}EXPORT_SYMBOL_GPL(percpu_ref_reinit);