#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 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);
