percpu-refcount: implement percpu_ref_reinit() and percpu_ref_is_zero()

Now that explicit invocation of percpu_ref_exit() is necessary to free
the percpu counter, we can implement percpu_ref_reinit() which
reinitializes a released percpu_ref.  This can be used implement
scalable gating switch which can be drained and then re-opened without
worrying about memory allocation failures.

percpu_ref_is_zero() is added to be used in a sanity check in
percpu_ref_exit().  As this function will be useful for other purposes
too, make it a public interface.

v2: Use smp_read_barrier_depends() instead of smp_load_acquire().  We
    only need data dep barrier and smp_load_acquire() is stronger and
    heavier on some archs.  Spotted by Lai Jiangshan.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Kent Overstreet <kmo@daterainc.com>
Cc: Christoph Lameter <cl@linux-foundation.org>
Cc: Lai Jiangshan <laijs@cn.fujitsu.com>

diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
index 0ddd283..3dfbf23 100644 (file)
--- 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 ac42991..fe5a334 100644 (file)
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -60,6 +60,41 @@ 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