#define pr_fmt(fmt) "%s: " fmt "\n", __func__ #include <linux/kernel.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 moduler arithmatic is commutative the sum of all the * pcpu_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_t * can't hit 0 before we've added up all the percpu refs. */ #define PCPU_COUNT_BIAS		(1U << 31) /** * percpu_ref_init - initialize a percpu refcount * @ref: percpu_ref to initialize * @release: function which will be called when refcount hits 0 * * Initializes the refcount in single atomic counter mode with a refcount of 1; * analagous to atomic_set(ref, 1). * * 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){	atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS); 	ref->pcpu_count = alloc_percpu(unsigned);	if (!ref->pcpu_count)		return -ENOMEM; 	ref->release = release;	return 0;}EXPORT_SYMBOL_GPL(percpu_ref_init); /** * percpu_ref_cancel_init - cancel percpu_ref_init() * @ref: percpu_ref to cancel init for * * Once a percpu_ref is initialized, its destruction is initiated by * percpu_ref_kill() and completes asynchronously, which can be painful to * do when destroying a half-constructed object in init failure path. * * This function destroys @ref without invoking @ref->release and the * memory area containing it can be freed immediately on return.  To * prevent accidental misuse, it's required that @ref has finished * percpu_ref_init(), whether successful or not, but never used. * * The weird name and usage restriction are to prevent people from using * this function by mistake for normal shutdown instead of * percpu_ref_kill(). */void percpu_ref_cancel_init(struct percpu_ref *ref){	unsigned __percpu *pcpu_count = ref->pcpu_count;	int cpu; 	WARN_ON_ONCE(atomic_read(&ref->count) != 1 + PCPU_COUNT_BIAS); 	if (pcpu_count) {		for_each_possible_cpu(cpu)			WARN_ON_ONCE(*per_cpu_ptr(pcpu_count, cpu));		free_percpu(ref->pcpu_count);	}}EXPORT_SYMBOL_GPL(percpu_ref_cancel_init); static void percpu_ref_kill_rcu(struct rcu_head *rcu){	struct percpu_ref *ref = container_of(rcu, struct percpu_ref, rcu);	unsigned __percpu *pcpu_count = ref->pcpu_count;	unsigned count = 0;	int cpu; 	/* Mask out PCPU_REF_DEAD */	pcpu_count = (unsigned __percpu *)		(((unsigned long) pcpu_count) & ~PCPU_STATUS_MASK); 	for_each_possible_cpu(cpu)		count += *per_cpu_ptr(pcpu_count, cpu); 	free_percpu(pcpu_count); 	pr_debug("global %i pcpu %i", atomic_read(&ref->count), (int) 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_add((int) count - PCPU_COUNT_BIAS, &ref->count); 	/* @ref is viewed as dead on all CPUs, send out kill confirmation */	if (ref->confirm_kill)		ref->confirm_kill(ref); 	/*	 * Now we're in single atomic_t mode with a consistent refcount, so it's	 * safe to drop our initial ref:	 */	percpu_ref_put(ref);} /** * 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 - all further * invocations of percpu_ref_tryget() will fail.  See percpu_ref_tryget() * for more details. * * Due to the way percpu_ref is implemented, @confirm_kill will be called * after at least one full RCU grace period has passed but this is an * implementation detail and callers must not depend on it. */void percpu_ref_kill_and_confirm(struct percpu_ref *ref,				 percpu_ref_func_t *confirm_kill){	WARN_ONCE(REF_STATUS(ref->pcpu_count) == PCPU_REF_DEAD,		  "percpu_ref_kill() called more than once!\n"); 	ref->pcpu_count = (unsigned __percpu *)		(((unsigned long) ref->pcpu_count)|PCPU_REF_DEAD);	ref->confirm_kill = confirm_kill; 	call_rcu_sched(&ref->rcu, percpu_ref_kill_rcu);}EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);