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-rw-r--r--lib/percpu-refcount.c478
1 files changed, 478 insertions, 0 deletions
diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
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--- /dev/null
+++ b/lib/percpu-refcount.c
@@ -0,0 +1,478 @@
+// SPDX-License-Identifier: GPL-2.0-only
+#define pr_fmt(fmt) "%s: " fmt, __func__
+
+#include <linux/kernel.h>
+#include <linux/sched.h>
+#include <linux/wait.h>
+#include <linux/slab.h>
+#include <linux/mm.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 DEFINE_SPINLOCK(percpu_ref_switch_lock);
+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. @ref starts out in percpu mode with a refcount of 1 unless
+ * @flags contains PERCPU_REF_INIT_ATOMIC or PERCPU_REF_INIT_DEAD. These flags
+ * change the start state to atomic with the latter setting the initial refcount
+ * to 0. 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;
+ struct percpu_ref_data *data;
+
+ ref->percpu_count_ptr = (unsigned long)
+ __alloc_percpu_gfp(sizeof(unsigned long), align, gfp);
+ if (!ref->percpu_count_ptr)
+ return -ENOMEM;
+
+ data = kzalloc(sizeof(*ref->data), gfp);
+ if (!data) {
+ free_percpu((void __percpu *)ref->percpu_count_ptr);
+ ref->percpu_count_ptr = 0;
+ return -ENOMEM;
+ }
+
+ data->force_atomic = flags & PERCPU_REF_INIT_ATOMIC;
+ data->allow_reinit = flags & PERCPU_REF_ALLOW_REINIT;
+
+ if (flags & (PERCPU_REF_INIT_ATOMIC | PERCPU_REF_INIT_DEAD)) {
+ ref->percpu_count_ptr |= __PERCPU_REF_ATOMIC;
+ data->allow_reinit = true;
+ } 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(&data->count, start_count);
+
+ data->release = release;
+ data->confirm_switch = NULL;
+ data->ref = ref;
+ ref->data = data;
+ return 0;
+}
+EXPORT_SYMBOL_GPL(percpu_ref_init);
+
+static void __percpu_ref_exit(struct percpu_ref *ref)
+{
+ unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
+
+ if (percpu_count) {
+ /* non-NULL confirm_switch indicates switching in progress */
+ WARN_ON_ONCE(ref->data && ref->data->confirm_switch);
+ free_percpu(percpu_count);
+ ref->percpu_count_ptr = __PERCPU_REF_ATOMIC_DEAD;
+ }
+}
+
+/**
+ * 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)
+{
+ struct percpu_ref_data *data = ref->data;
+ unsigned long flags;
+
+ __percpu_ref_exit(ref);
+
+ if (!data)
+ return;
+
+ spin_lock_irqsave(&percpu_ref_switch_lock, flags);
+ ref->percpu_count_ptr |= atomic_long_read(&ref->data->count) <<
+ __PERCPU_REF_FLAG_BITS;
+ ref->data = NULL;
+ spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
+
+ kfree(data);
+}
+EXPORT_SYMBOL_GPL(percpu_ref_exit);
+
+static void percpu_ref_call_confirm_rcu(struct rcu_head *rcu)
+{
+ struct percpu_ref_data *data = container_of(rcu,
+ struct percpu_ref_data, rcu);
+ struct percpu_ref *ref = data->ref;
+
+ data->confirm_switch(ref);
+ data->confirm_switch = NULL;
+ wake_up_all(&percpu_ref_switch_waitq);
+
+ if (!data->allow_reinit)
+ __percpu_ref_exit(ref);
+
+ /* 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_data *data = container_of(rcu,
+ struct percpu_ref_data, rcu);
+ struct percpu_ref *ref = data->ref;
+ unsigned long __percpu *percpu_count = percpu_count_ptr(ref);
+ static atomic_t underflows;
+ unsigned long count = 0;
+ int cpu;
+
+ for_each_possible_cpu(cpu)
+ count += *per_cpu_ptr(percpu_count, cpu);
+
+ pr_debug("global %lu percpu %lu\n",
+ atomic_long_read(&data->count), 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, &data->count);
+
+ if (WARN_ONCE(atomic_long_read(&data->count) <= 0,
+ "percpu ref (%ps) <= 0 (%ld) after switching to atomic",
+ data->release, atomic_long_read(&data->count)) &&
+ atomic_inc_return(&underflows) < 4) {
+ pr_err("%s(): percpu_ref underflow", __func__);
+ mem_dump_obj(data);
+ }
+
+ /* @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) {
+ if (confirm_switch)
+ confirm_switch(ref);
+ return;
+ }
+
+ /* 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.
+ */
+ ref->data->confirm_switch = confirm_switch ?:
+ percpu_ref_noop_confirm_switch;
+
+ percpu_ref_get(ref); /* put after confirmation */
+ call_rcu(&ref->data->rcu, percpu_ref_switch_to_atomic_rcu);
+}
+
+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;
+
+ if (WARN_ON_ONCE(!ref->data->allow_reinit))
+ return;
+
+ atomic_long_add(PERCPU_COUNT_BIAS, &ref->data->count);
+
+ /*
+ * Restore per-cpu operation. smp_store_release() is paired
+ * with READ_ONCE() 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);
+}
+
+static void __percpu_ref_switch_mode(struct percpu_ref *ref,
+ percpu_ref_func_t *confirm_switch)
+{
+ struct percpu_ref_data *data = ref->data;
+
+ lockdep_assert_held(&percpu_ref_switch_lock);
+
+ /*
+ * If the previous ATOMIC switching hasn't finished yet, wait for
+ * its completion. If the caller ensures that ATOMIC switching
+ * isn't in progress, this function can be called from any context.
+ */
+ wait_event_lock_irq(percpu_ref_switch_waitq, !data->confirm_switch,
+ percpu_ref_switch_lock);
+
+ if (data->force_atomic || percpu_ref_is_dying(ref))
+ __percpu_ref_switch_to_atomic(ref, confirm_switch);
+ else
+ __percpu_ref_switch_to_percpu(ref);
+}
+
+/**
+ * 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 may block if @ref is in the process of switching to atomic
+ * mode. If the caller ensures that @ref is not in the process of
+ * switching to atomic mode, this function can be called from any context.
+ */
+void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
+ percpu_ref_func_t *confirm_switch)
+{
+ unsigned long flags;
+
+ spin_lock_irqsave(&percpu_ref_switch_lock, flags);
+
+ ref->data->force_atomic = true;
+ __percpu_ref_switch_mode(ref, confirm_switch);
+
+ spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
+}
+EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic);
+
+/**
+ * percpu_ref_switch_to_atomic_sync - switch a percpu_ref to atomic mode
+ * @ref: percpu_ref to switch to atomic mode
+ *
+ * Schedule switching the ref to atomic mode, and wait for the
+ * switch to complete. Caller must ensure that no other thread
+ * will switch back to percpu mode.
+ */
+void percpu_ref_switch_to_atomic_sync(struct percpu_ref *ref)
+{
+ percpu_ref_switch_to_atomic(ref, NULL);
+ wait_event(percpu_ref_switch_waitq, !ref->data->confirm_switch);
+}
+EXPORT_SYMBOL_GPL(percpu_ref_switch_to_atomic_sync);
+
+/**
+ * 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 may block if @ref is in the process of switching to atomic
+ * mode. If the caller ensures that @ref is not in the process of
+ * switching to atomic mode, this function can be called from any context.
+ */
+void percpu_ref_switch_to_percpu(struct percpu_ref *ref)
+{
+ unsigned long flags;
+
+ spin_lock_irqsave(&percpu_ref_switch_lock, flags);
+
+ ref->data->force_atomic = false;
+ __percpu_ref_switch_mode(ref, NULL);
+
+ spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
+}
+EXPORT_SYMBOL_GPL(percpu_ref_switch_to_percpu);
+
+/**
+ * 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_to_atomic().
+ *
+ * There are no implied RCU grace periods between kill and release.
+ */
+void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
+ percpu_ref_func_t *confirm_kill)
+{
+ unsigned long flags;
+
+ spin_lock_irqsave(&percpu_ref_switch_lock, flags);
+
+ WARN_ONCE(percpu_ref_is_dying(ref),
+ "%s called more than once on %ps!", __func__,
+ ref->data->release);
+
+ ref->percpu_count_ptr |= __PERCPU_REF_DEAD;
+ __percpu_ref_switch_mode(ref, confirm_kill);
+ percpu_ref_put(ref);
+
+ spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
+}
+EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
+
+/**
+ * percpu_ref_is_zero - test whether a percpu refcount reached zero
+ * @ref: percpu_ref to test
+ *
+ * Returns %true if @ref reached zero.
+ *
+ * This function is safe to call as long as @ref is between init and exit.
+ */
+bool percpu_ref_is_zero(struct percpu_ref *ref)
+{
+ unsigned long __percpu *percpu_count;
+ unsigned long count, flags;
+
+ if (__ref_is_percpu(ref, &percpu_count))
+ return false;
+
+ /* protect us from being destroyed */
+ spin_lock_irqsave(&percpu_ref_switch_lock, flags);
+ if (ref->data)
+ count = atomic_long_read(&ref->data->count);
+ else
+ count = ref->percpu_count_ptr >> __PERCPU_REF_FLAG_BITS;
+ spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
+
+ return count == 0;
+}
+EXPORT_SYMBOL_GPL(percpu_ref_is_zero);
+
+/**
+ * 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));
+
+ percpu_ref_resurrect(ref);
+}
+EXPORT_SYMBOL_GPL(percpu_ref_reinit);
+
+/**
+ * percpu_ref_resurrect - modify a percpu refcount from dead to live
+ * @ref: perpcu_ref to resurrect
+ *
+ * Modify @ref so that it's in the same state as before percpu_ref_kill() was
+ * called. @ref must be dead but must not yet have exited.
+ *
+ * If @ref->release() frees @ref then the caller is responsible for
+ * guaranteeing that @ref->release() does not get called while this
+ * function is in progress.
+ *
+ * Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
+ * this function is in progress.
+ */
+void percpu_ref_resurrect(struct percpu_ref *ref)
+{
+ unsigned long __percpu *percpu_count;
+ unsigned long flags;
+
+ spin_lock_irqsave(&percpu_ref_switch_lock, flags);
+
+ WARN_ON_ONCE(!percpu_ref_is_dying(ref));
+ WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count));
+
+ ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
+ percpu_ref_get(ref);
+ __percpu_ref_switch_mode(ref, NULL);
+
+ spin_unlock_irqrestore(&percpu_ref_switch_lock, flags);
+}
+EXPORT_SYMBOL_GPL(percpu_ref_resurrect);