#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) static unsigned __percpu *pcpu_count_ptr(struct percpu_ref *ref){	return (unsigned __percpu *)(ref->pcpu_count_ptr & ~PCPU_REF_DEAD);} /** * 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_ptr = (unsigned long)alloc_percpu(unsigned);	if (!ref->pcpu_count_ptr)		return -ENOMEM; 	ref->release = release;	return 0;}EXPORT_SYMBOL_GPL(percpu_ref_init); /** * 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().  @ref must have been initialized successfully, killed * 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 __percpu *pcpu_count = pcpu_count_ptr(ref);	int cpu; 	BUG_ON(!pcpu_count);	WARN_ON(!percpu_ref_is_zero(ref)); 	atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS); 	/*	 * Restore per-cpu operation.  smp_store_release() is paired with	 * smp_read_barrier_depends() in __pcpu_ref_alive() and guarantees	 * that the zeroing is visible to all percpu accesses which can see	 * the following PCPU_REF_DEAD clearing.	 */	for_each_possible_cpu(cpu)		*per_cpu_ptr(pcpu_count, cpu) = 0; 	smp_store_release(&ref->pcpu_count_ptr,			  ref->pcpu_count_ptr & ~PCPU_REF_DEAD);}EXPORT_SYMBOL_GPL(percpu_ref_reinit); /** * 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 __percpu *pcpu_count = pcpu_count_ptr(ref); 	if (pcpu_count) {		free_percpu(pcpu_count);		ref->pcpu_count_ptr = PCPU_REF_DEAD;	}}EXPORT_SYMBOL_GPL(percpu_ref_exit); 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 = pcpu_count_ptr(ref);	unsigned count = 0;	int cpu; 	for_each_possible_cpu(cpu)		count += *per_cpu_ptr(pcpu_count, cpu); 	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); 	WARN_ONCE(atomic_read(&ref->count) <= 0, "percpu ref <= 0 (%i)",		  atomic_read(&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->pcpu_count_ptr & PCPU_REF_DEAD,		  "percpu_ref_kill() called more than once!\n"); 	ref->pcpu_count_ptr |= 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); /* * XXX: Temporary kludge to work around SCSI blk-mq stall.  Used only by * block/blk-mq.c::blk_mq_freeze_queue().  Will be removed during v3.18 * devel cycle.  Do not use anywhere else. */void __percpu_ref_kill_expedited(struct percpu_ref *ref){	WARN_ONCE(ref->pcpu_count_ptr & PCPU_REF_DEAD,		  "percpu_ref_kill() called more than once on %pf!",		  ref->release); 	ref->pcpu_count_ptr |= PCPU_REF_DEAD;	synchronize_sched_expedited();	percpu_ref_kill_rcu(&ref->rcu);}