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 <t...@kernel.org>
Cc: Kent Overstreet <k...@daterainc.com>
Cc: Christoph Lameter <c...@linux-foundation.org>
Cc: Lai Jiangshan <la...@cn.fujitsu.com>
---
include/linux/percpu-refcount.h | 19 +++++++++++++++++++
lib/percpu-refcount.c | 35 +++++++++++++++++++++++++++++++++++
2 files changed, 54 insertions(+)
--- 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(stru
{
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
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
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -61,6 +61,41 @@ int percpu_ref_init(struct percpu_ref *r
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_load_acquire() 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
*
--
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