diff options
| -rw-r--r-- | include/linux/percpu-refcount.h | 122 | ||||
| -rw-r--r-- | lib/Makefile | 2 | ||||
| -rw-r--r-- | lib/percpu-refcount.c | 128 |
3 files changed, 251 insertions, 1 deletions
diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h new file mode 100644 index 00000000000..24b31ef1593 --- /dev/null +++ b/include/linux/percpu-refcount.h @@ -0,0 +1,122 @@ +/* + * Percpu refcounts: + * (C) 2012 Google, Inc. + * Author: Kent Overstreet <koverstreet@google.com> + * + * This implements a refcount with similar semantics to atomic_t - atomic_inc(), + * atomic_dec_and_test() - but percpu. + * + * There's one important difference between percpu refs and normal atomic_t + * refcounts; you have to keep track of your initial refcount, and then when you + * start shutting down you call percpu_ref_kill() _before_ dropping the initial + * refcount. + * + * The refcount will have a range of 0 to ((1U << 31) - 1), i.e. one bit less + * than an atomic_t - this is because of the way shutdown works, see + * percpu_ref_kill()/PCPU_COUNT_BIAS. + * + * Before you call percpu_ref_kill(), percpu_ref_put() does not check for the + * refcount hitting 0 - it can't, if it was in percpu mode. percpu_ref_kill() + * puts the ref back in single atomic_t mode, collecting the per cpu refs and + * issuing the appropriate barriers, and then marks the ref as shutting down so + * that percpu_ref_put() will check for the ref hitting 0. After it returns, + * it's safe to drop the initial ref. + * + * USAGE: + * + * See fs/aio.c for some example usage; it's used there for struct kioctx, which + * is created when userspaces calls io_setup(), and destroyed when userspace + * calls io_destroy() or the process exits. + * + * In the aio code, kill_ioctx() is called when we wish to destroy a kioctx; it + * calls percpu_ref_kill(), then hlist_del_rcu() and sychronize_rcu() to remove + * the kioctx from the proccess's list of kioctxs - after that, there can't be + * any new users of the kioctx (from lookup_ioctx()) and it's then safe to drop + * the initial ref with percpu_ref_put(). + * + * Code that does a two stage shutdown like this often needs some kind of + * explicit synchronization to ensure the initial refcount can only be dropped + * once - percpu_ref_kill() does this for you, it returns true once and false if + * someone else already called it. The aio code uses it this way, but it's not + * necessary if the code has some other mechanism to synchronize teardown. + * around. + */ + +#ifndef _LINUX_PERCPU_REFCOUNT_H +#define _LINUX_PERCPU_REFCOUNT_H + +#include <linux/atomic.h> +#include <linux/kernel.h> +#include <linux/percpu.h> +#include <linux/rcupdate.h> + +struct percpu_ref; +typedef void (percpu_ref_release)(struct percpu_ref *); + +struct percpu_ref { + atomic_t count; + /* + * The low bit of the pointer indicates whether the ref is in percpu + * mode; if set, then get/put will manipulate the atomic_t (this is a + * hack because we need to keep the pointer around for + * percpu_ref_kill_rcu()) + */ + unsigned __percpu *pcpu_count; + percpu_ref_release *release; + struct rcu_head rcu; +}; + +int percpu_ref_init(struct percpu_ref *, percpu_ref_release *); +void percpu_ref_kill(struct percpu_ref *ref); + +#define PCPU_STATUS_BITS 2 +#define PCPU_STATUS_MASK ((1 << PCPU_STATUS_BITS) - 1) +#define PCPU_REF_PTR 0 +#define PCPU_REF_DEAD 1 + +#define REF_STATUS(count) (((unsigned long) count) & PCPU_STATUS_MASK) + +/** + * percpu_ref_get - increment a percpu refcount + * + * Analagous to atomic_inc(). + */ +static inline void percpu_ref_get(struct percpu_ref *ref) +{ + unsigned __percpu *pcpu_count; + + preempt_disable(); + + pcpu_count = ACCESS_ONCE(ref->pcpu_count); + + if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) + __this_cpu_inc(*pcpu_count); + else + atomic_inc(&ref->count); + + preempt_enable(); +} + +/** + * percpu_ref_put - decrement a percpu refcount + * + * Decrement the refcount, and if 0, call the release function (which was passed + * to percpu_ref_init()) + */ +static inline void percpu_ref_put(struct percpu_ref *ref) +{ + unsigned __percpu *pcpu_count; + + preempt_disable(); + + pcpu_count = ACCESS_ONCE(ref->pcpu_count); + + if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) + __this_cpu_dec(*pcpu_count); + else if (unlikely(atomic_dec_and_test(&ref->count))) + ref->release(ref); + + preempt_enable(); +} + +#endif diff --git a/lib/Makefile b/lib/Makefile index c55a037a354..386db4bbc26 100644 --- a/lib/Makefile +++ b/lib/Makefile @@ -13,7 +13,7 @@ lib-y := ctype.o string.o vsprintf.o cmdline.o \ sha1.o md5.o irq_regs.o reciprocal_div.o argv_split.o \ proportions.o flex_proportions.o prio_heap.o ratelimit.o show_mem.o \ is_single_threaded.o plist.o decompress.o kobject_uevent.o \ - earlycpio.o + earlycpio.o percpu-refcount.o obj-$(CONFIG_ARCH_HAS_DEBUG_STRICT_USER_COPY_CHECKS) += usercopy.o lib-$(CONFIG_MMU) += ioremap.o diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c new file mode 100644 index 00000000000..6f0ffd702a0 --- /dev/null +++ b/lib/percpu-refcount.c @@ -0,0 +1,128 @@ +#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: 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_release *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; +} + +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; + unsigned count = 0; + int cpu; + + pcpu_count = ACCESS_ONCE(ref->pcpu_count); + + /* 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); + + /* + * 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 - safely drop initial ref + * + * Must be used to drop the initial ref on a percpu refcount; must be called + * precisely once before shutdown. + * + * Puts @ref in non percpu mode, then does a call_rcu() before gathering up the + * percpu counters and dropping the initial ref. + */ +void percpu_ref_kill(struct percpu_ref *ref) +{ + unsigned __percpu *pcpu_count, *old, *new; + + pcpu_count = ACCESS_ONCE(ref->pcpu_count); + + do { + if (REF_STATUS(pcpu_count) == PCPU_REF_DEAD) { + WARN(1, "percpu_ref_kill() called more than once!\n"); + return; + } + + old = pcpu_count; + new = (unsigned __percpu *) + (((unsigned long) pcpu_count)|PCPU_REF_DEAD); + + pcpu_count = cmpxchg(&ref->pcpu_count, old, new); + } while (pcpu_count != old); + + call_rcu(&ref->rcu, percpu_ref_kill_rcu); +} |