1 files changed, 371 insertions, 0 deletions
diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
new file mode 100644
index 000000000..9f96fa7bc
--- /dev/null
+++ b/lib/percpu-refcount.c
@@ -0,0 +1,371 @@
+#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 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.  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;
+	ref->confirm_switch = NULL;
+	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) {
+		/* non-NULL confirm_switch indicates switching in progress */
+		WARN_ON_ONCE(ref->confirm_switch);
+		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) {
+		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->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);
+}
+
+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;
+
+	atomic_long_add(PERCPU_COUNT_BIAS, &ref->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)
+{
+	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, !ref->confirm_switch,
+			    percpu_ref_switch_lock);
+
+	if (ref->force_atomic || (ref->percpu_count_ptr & __PERCPU_REF_DEAD))
+		__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->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->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->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(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_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_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)
+{
+	unsigned long flags;
+
+	spin_lock_irqsave(&percpu_ref_switch_lock, flags);
+
+	WARN_ON_ONCE(!percpu_ref_is_zero(ref));
+
+	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_reinit);