2 files changed, 54 insertions, 0 deletions

diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
index 0ddd2839ca84..3dfbf237cd8f 100644
--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -67,6 +67,7 @@ struct percpu_ref {
 
 int __must_check percpu_ref_init(struct percpu_ref *ref,
                                  percpu_ref_func_t *release);
+void percpu_ref_reinit(struct percpu_ref *ref);
 void percpu_ref_exit(struct percpu_ref *ref);
 void percpu_ref_kill_and_confirm(struct percpu_ref *ref,
                                  percpu_ref_func_t *confirm_kill);
@@ -99,6 +100,9 @@ static inline bool __pcpu_ref_alive(struct percpu_ref *ref,
 {
         unsigned long pcpu_ptr = ACCESS_ONCE(ref->pcpu_count_ptr);
 
+        /* paired with smp_store_release() in percpu_ref_reinit() */
+        smp_read_barrier_depends();
+
         if (unlikely(pcpu_ptr & PCPU_REF_DEAD))
                 return false;
 
@@ -206,4 +210,19 @@ static inline void percpu_ref_put(struct percpu_ref *ref)
         rcu_read_unlock_sched();
 }
 
+/**
+ * percpu_ref_is_zero - test whether a percpu refcount reached zero
+ * @ref: percpu_ref to test
+ *
+ * Returns %true if @ref reached zero.
+ */
+static inline bool percpu_ref_is_zero(struct percpu_ref *ref)
+{
+        unsigned __percpu *pcpu_count;
+
+        if (__pcpu_ref_alive(ref, &pcpu_count))
+                return false;
+        return !atomic_read(&ref->count);
+}
+
 #endif
diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index ac4299120087..fe5a3342e960 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -61,6 +61,41 @@ int percpu_ref_init(struct percpu_ref *ref, percpu_ref_func_t *release)
 EXPORT_SYMBOL_GPL(percpu_ref_init);
 
 /**
+ * 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().  @ref must have been initialized successfully, killed
+ * 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 __percpu *pcpu_count = pcpu_count_ptr(ref);
+        int cpu;
+
+        BUG_ON(!pcpu_count);
+        WARN_ON(!percpu_ref_is_zero(ref));
+
+        atomic_set(&ref->count, 1 + PCPU_COUNT_BIAS);
+
+        /*
+         * Restore per-cpu operation.  smp_store_release() is paired with
+         * smp_read_barrier_depends() in __pcpu_ref_alive() and guarantees
+         * that the zeroing is visible to all percpu accesses which can see
+         * the following PCPU_REF_DEAD clearing.
+         */
+        for_each_possible_cpu(cpu)
+                *per_cpu_ptr(pcpu_count, cpu) = 0;
+
+        smp_store_release(&ref->pcpu_count_ptr,
+                          ref->pcpu_count_ptr & ~PCPU_REF_DEAD);
+}
+EXPORT_SYMBOL_GPL(percpu_ref_reinit);
+
+/**
  * percpu_ref_exit - undo percpu_ref_init()
  * @ref: percpu_ref to exit
  *