| #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: 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 = alloc_percpu(unsigned); |
| if (!ref->pcpu_count) |
| return -ENOMEM; |
| |
| ref->release = release; |
| return 0; |
| } |
| EXPORT_SYMBOL_GPL(percpu_ref_init); |
| |
| /** |
| * percpu_ref_cancel_init - cancel percpu_ref_init() |
| * @ref: percpu_ref to cancel init for |
| * |
| * Once a percpu_ref is initialized, its destruction is initiated by |
| * percpu_ref_kill() and completes asynchronously, which can be painful to |
| * do when destroying a half-constructed object in init failure path. |
| * |
| * This function destroys @ref without invoking @ref->release and the |
| * memory area containing it can be freed immediately on return. To |
| * prevent accidental misuse, it's required that @ref has finished |
| * percpu_ref_init(), whether successful or not, but never used. |
| * |
| * The weird name and usage restriction are to prevent people from using |
| * this function by mistake for normal shutdown instead of |
| * percpu_ref_kill(). |
| */ |
| void percpu_ref_cancel_init(struct percpu_ref *ref) |
| { |
| unsigned __percpu *pcpu_count = ref->pcpu_count; |
| int cpu; |
| |
| WARN_ON_ONCE(atomic_read(&ref->count) != 1 + PCPU_COUNT_BIAS); |
| |
| if (pcpu_count) { |
| for_each_possible_cpu(cpu) |
| WARN_ON_ONCE(*per_cpu_ptr(pcpu_count, cpu)); |
| free_percpu(ref->pcpu_count); |
| } |
| } |
| EXPORT_SYMBOL_GPL(percpu_ref_cancel_init); |
| |
| 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 = ref->pcpu_count; |
| unsigned count = 0; |
| int cpu; |
| |
| /* Mask out PCPU_REF_DEAD */ |
| pcpu_count = (unsigned __percpu *) |
| (((unsigned long) pcpu_count) & ~PCPU_REF_DEAD); |
| |
| 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); |
| |
| 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_STATUS(ref->pcpu_count) == PCPU_REF_DEAD, |
| "percpu_ref_kill() called more than once!\n"); |
| |
| ref->pcpu_count = (unsigned __percpu *) |
| (((unsigned long) ref->pcpu_count)|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); |