#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);
}
