diff options
Diffstat (limited to 'lib/percpu-refcount.c')
| -rw-r--r-- | lib/percpu-refcount.c | 331 |
1 files changed, 331 insertions, 0 deletions
diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c new file mode 100644 index 000000000..6111bcb28 --- /dev/null +++ b/lib/percpu-refcount.c @@ -0,0 +1,331 @@ +#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 moduler arithmatic 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 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; + 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) { + 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)) { + /* 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. + */ + WARN_ON_ONCE(ref->confirm_switch); + 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); + } else if (confirm_switch) { + /* + * Somebody already set ATOMIC. Switching may still be in + * progress. @confirm_switch must be invoked after the + * switching is complete and a full sched RCU grace period + * has passed. Wait synchronously for the previous + * switching and schedule @confirm_switch invocation. + */ + wait_event(percpu_ref_switch_waitq, !ref->confirm_switch); + ref->confirm_switch = confirm_switch; + + percpu_ref_get(ref); /* put after confirmation */ + call_rcu_sched(&ref->rcu, percpu_ref_call_confirm_rcu); + } +} + +/** + * 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 normally doesn't block and can be called from any context + * but it may block if @confirm_kill is specified and @ref is already in + * the process of switching to atomic mode. In such cases, @confirm_switch + * will be invoked after the switching is complete. + * + * Due to the way percpu_ref is implemented, @confirm_switch will be called + * after at least one full sched RCU grace period has passed but this is an + * implementation detail and must not be depended upon. + */ +void percpu_ref_switch_to_atomic(struct percpu_ref *ref, + percpu_ref_func_t *confirm_switch) +{ + ref->force_atomic = true; + __percpu_ref_switch_to_atomic(ref, confirm_switch); +} + +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; + + wait_event(percpu_ref_switch_waitq, !ref->confirm_switch); + + 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); +} + +/** + * 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 normally doesn't block and can be called from any context + * but it may block if @ref is in the process of switching to atomic mode + * by percpu_ref_switch_atomic(). + */ +void percpu_ref_switch_to_percpu(struct percpu_ref *ref) +{ + ref->force_atomic = false; + + /* a dying or dead ref can't be switched to percpu mode w/o reinit */ + if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) + __percpu_ref_switch_to_percpu(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 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_atomic(). + * + * Due to the way percpu_ref is implemented, @confirm_switch will be called + * after at least one full sched RCU grace period has passed but this is an + * implementation detail and must not be depended upon. + */ +void percpu_ref_kill_and_confirm(struct percpu_ref *ref, + percpu_ref_func_t *confirm_kill) +{ + 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_to_atomic(ref, confirm_kill); + percpu_ref_put(ref); +} +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) +{ + WARN_ON_ONCE(!percpu_ref_is_zero(ref)); + + ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD; + percpu_ref_get(ref); + if (!ref->force_atomic) + __percpu_ref_switch_to_percpu(ref); +} +EXPORT_SYMBOL_GPL(percpu_ref_reinit); |