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diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
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+/*
+ * 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()/PERCPU_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 synchronize_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>
+#include <linux/gfp.h>
+
+struct percpu_ref;
+typedef void (percpu_ref_func_t)(struct percpu_ref *);
+
+/* flags set in the lower bits of percpu_ref->percpu_count_ptr */
+enum {
+	__PERCPU_REF_ATOMIC	= 1LU << 0,	/* operating in atomic mode */
+	__PERCPU_REF_DEAD	= 1LU << 1,	/* (being) killed */
+	__PERCPU_REF_ATOMIC_DEAD = __PERCPU_REF_ATOMIC | __PERCPU_REF_DEAD,
+
+	__PERCPU_REF_FLAG_BITS	= 2,
+};
+
+/* @flags for percpu_ref_init() */
+enum {
+	/*
+	 * Start w/ ref == 1 in atomic mode.  Can be switched to percpu
+	 * operation using percpu_ref_switch_to_percpu().  If initialized
+	 * with this flag, the ref will stay in atomic mode until
+	 * percpu_ref_switch_to_percpu() is invoked on it.
+	 */
+	PERCPU_REF_INIT_ATOMIC	= 1 << 0,
+
+	/*
+	 * Start dead w/ ref == 0 in atomic mode.  Must be revived with
+	 * percpu_ref_reinit() before used.  Implies INIT_ATOMIC.
+	 */
+	PERCPU_REF_INIT_DEAD	= 1 << 1,
+};
+
+struct percpu_ref {
+	atomic_long_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.
+	 */
+	unsigned long		percpu_count_ptr;
+	percpu_ref_func_t	*release;
+	percpu_ref_func_t	*confirm_switch;
+	bool			force_atomic:1;
+	struct rcu_head		rcu;
+};
+
+int __must_check percpu_ref_init(struct percpu_ref *ref,
+				 percpu_ref_func_t *release, unsigned int flags,
+				 gfp_t gfp);
+void percpu_ref_exit(struct percpu_ref *ref);
+void percpu_ref_switch_to_atomic(struct percpu_ref *ref,
+				 percpu_ref_func_t *confirm_switch);
+void percpu_ref_switch_to_percpu(struct percpu_ref *ref);
+void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
+				 percpu_ref_func_t *confirm_kill);
+void percpu_ref_reinit(struct percpu_ref *ref);
+
+/**
+ * percpu_ref_kill - drop the initial ref
+ * @ref: percpu_ref to kill
+ *
+ * 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.
+ */
+static inline void percpu_ref_kill(struct percpu_ref *ref)
+{
+	return percpu_ref_kill_and_confirm(ref, NULL);
+}
+
+/*
+ * Internal helper.  Don't use outside percpu-refcount proper.  The
+ * function doesn't return the pointer and let the caller test it for NULL
+ * because doing so forces the compiler to generate two conditional
+ * branches as it can't assume that @ref->percpu_count is not NULL.
+ */
+static inline bool __ref_is_percpu(struct percpu_ref *ref,
+					  unsigned long __percpu **percpu_countp)
+{
+	unsigned long percpu_ptr;
+
+	/*
+	 * The value of @ref->percpu_count_ptr is tested for
+	 * !__PERCPU_REF_ATOMIC, which may be set asynchronously, and then
+	 * used as a pointer.  If the compiler generates a separate fetch
+	 * when using it as a pointer, __PERCPU_REF_ATOMIC may be set in
+	 * between contaminating the pointer value, meaning that
+	 * ACCESS_ONCE() is required when fetching it.
+	 *
+	 * Also, we need a data dependency barrier to be paired with
+	 * smp_store_release() in __percpu_ref_switch_to_percpu().
+	 *
+	 * Use lockless deref which contains both.
+	 */
+	percpu_ptr = lockless_dereference(ref->percpu_count_ptr);
+
+	/*
+	 * Theoretically, the following could test just ATOMIC; however,
+	 * then we'd have to mask off DEAD separately as DEAD may be
+	 * visible without ATOMIC if we race with percpu_ref_kill().  DEAD
+	 * implies ATOMIC anyway.  Test them together.
+	 */
+	if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
+		return false;
+
+	*percpu_countp = (unsigned long __percpu *)percpu_ptr;
+	return true;
+}
+
+/**
+ * percpu_ref_get_many - increment a percpu refcount
+ * @ref: percpu_ref to get
+ * @nr: number of references to get
+ *
+ * Analogous to atomic_long_add().
+ *
+ * This function is safe to call as long as @ref is between init and exit.
+ */
+static inline void percpu_ref_get_many(struct percpu_ref *ref, unsigned long nr)
+{
+	unsigned long __percpu *percpu_count;
+
+	rcu_read_lock_sched();
+
+	if (__ref_is_percpu(ref, &percpu_count))
+		this_cpu_add(*percpu_count, nr);
+	else
+		atomic_long_add(nr, &ref->count);
+
+	rcu_read_unlock_sched();
+}
+
+/**
+ * percpu_ref_get - increment a percpu refcount
+ * @ref: percpu_ref to get
+ *
+ * Analagous to atomic_long_inc().
+ *
+ * This function is safe to call as long as @ref is between init and exit.
+ */
+static inline void percpu_ref_get(struct percpu_ref *ref)
+{
+	percpu_ref_get_many(ref, 1);
+}
+
+/**
+ * percpu_ref_tryget - try to increment a percpu refcount
+ * @ref: percpu_ref to try-get
+ *
+ * Increment a percpu refcount unless its count already reached zero.
+ * Returns %true on success; %false on failure.
+ *
+ * This function is safe to call as long as @ref is between init and exit.
+ */
+static inline bool percpu_ref_tryget(struct percpu_ref *ref)
+{
+	unsigned long __percpu *percpu_count;
+	int ret;
+
+	rcu_read_lock_sched();
+
+	if (__ref_is_percpu(ref, &percpu_count)) {
+		this_cpu_inc(*percpu_count);
+		ret = true;
+	} else {
+		ret = atomic_long_inc_not_zero(&ref->count);
+	}
+
+	rcu_read_unlock_sched();
+
+	return ret;
+}
+
+/**
+ * percpu_ref_tryget_live - try to increment a live percpu refcount
+ * @ref: percpu_ref to try-get
+ *
+ * Increment a percpu refcount unless it has already been killed.  Returns
+ * %true on success; %false on failure.
+ *
+ * Completion of percpu_ref_kill() in itself doesn't guarantee that this
+ * function will fail.  For such guarantee, percpu_ref_kill_and_confirm()
+ * should be used.  After the confirm_kill callback is invoked, it's
+ * guaranteed that no new reference will be given out by
+ * percpu_ref_tryget_live().
+ *
+ * This function is safe to call as long as @ref is between init and exit.
+ */
+static inline bool percpu_ref_tryget_live(struct percpu_ref *ref)
+{
+	unsigned long __percpu *percpu_count;
+	int ret = false;
+
+	rcu_read_lock_sched();
+
+	if (__ref_is_percpu(ref, &percpu_count)) {
+		this_cpu_inc(*percpu_count);
+		ret = true;
+	} else if (!(ref->percpu_count_ptr & __PERCPU_REF_DEAD)) {
+		ret = atomic_long_inc_not_zero(&ref->count);
+	}
+
+	rcu_read_unlock_sched();
+
+	return ret;
+}
+
+/**
+ * percpu_ref_put_many - decrement a percpu refcount
+ * @ref: percpu_ref to put
+ * @nr: number of references to put
+ *
+ * Decrement the refcount, and if 0, call the release function (which was passed
+ * to percpu_ref_init())
+ *
+ * This function is safe to call as long as @ref is between init and exit.
+ */
+static inline void percpu_ref_put_many(struct percpu_ref *ref, unsigned long nr)
+{
+	unsigned long __percpu *percpu_count;
+
+	rcu_read_lock_sched();
+
+	if (__ref_is_percpu(ref, &percpu_count))
+		this_cpu_sub(*percpu_count, nr);
+	else if (unlikely(atomic_long_sub_and_test(nr, &ref->count)))
+		ref->release(ref);
+
+	rcu_read_unlock_sched();
+}
+
+/**
+ * percpu_ref_put - decrement a percpu refcount
+ * @ref: percpu_ref to put
+ *
+ * Decrement the refcount, and if 0, call the release function (which was passed
+ * to percpu_ref_init())
+ *
+ * This function is safe to call as long as @ref is between init and exit.
+ */
+static inline void percpu_ref_put(struct percpu_ref *ref)
+{
+	percpu_ref_put_many(ref, 1);
+}
+
+/**
+ * percpu_ref_is_dying - test whether a percpu refcount is dying or dead
+ * @ref: percpu_ref to test
+ *
+ * Returns %true if @ref is dying or dead.
+ *
+ * This function is safe to call as long as @ref is between init and exit
+ * and the caller is responsible for synchronizing against state changes.
+ */
+static inline bool percpu_ref_is_dying(struct percpu_ref *ref)
+{
+	return ref->percpu_count_ptr & __PERCPU_REF_DEAD;
+}
+
+/**
+ * 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.
+ */
+static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
+{
+	unsigned long __percpu *percpu_count;
+
+	if (__ref_is_percpu(ref, &percpu_count))
+		return false;
+	return !atomic_long_read(&ref->count);
+}
+
+#endif