[PATCH] percpu-refcount: relax limit on percpu_ref_reinit()

Ming Lei
2018-09-09T17:48:13Z

Now percpu_ref_reinit() can only be done on one percpu refcounter
when it drops zero. And the limit shouldn't be so strict, and it
is quite straightforward that percpu_ref_reinit() can be done when
this counter is at atomic mode.

This patch relaxes the limit, so we may avoid extra change[1] for NVMe
timeout's requirement.

[1] https://marc.info/?l=linux-kernel&m=153612052611020&w=2

Cc: Tejun Heo <tj@kernel.org>
Cc: Jianchao Wang <jianchao.w.wang@oracle.com>
Cc: Kent Overstreet <kent.overstreet@gmail.com>
Cc: linux-block@vger.kernel.org
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
lib/percpu-refcount.c | 19 ++++++-------------
1 file changed, 6 insertions(+), 13 deletions(-)

diff --git a/lib/percpu-refcount.c b/lib/percpu-refcount.c
index 9f96fa7bc000..af6b514c7d72 100644
--- a/lib/percpu-refcount.c
+++ b/lib/percpu-refcount.c
@@ -130,8 +130,10 @@ static void percpu_ref_switch_to_atomic_rcu(struct rcu_head *rcu)
unsigned long count = 0;
int cpu;

- for_each_possible_cpu(cpu)
+ for_each_possible_cpu(cpu) {
count += *per_cpu_ptr(percpu_count, cpu);
+ *per_cpu_ptr(percpu_count, cpu) = 0;
+ }

pr_debug("global %ld percpu %ld",
atomic_long_read(&ref->count), (long)count);
@@ -187,7 +189,6 @@ static void __percpu_ref_switch_to_atomic(struct percpu_ref *ref,
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);

@@ -196,15 +197,6 @@ static void __percpu_ref_switch_to_percpu(struct percpu_ref *ref)

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);
}
@@ -349,7 +341,7 @@ EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
*
* 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.
+ * initialized successfully and in atomic mode but not exited.
*
* Note that percpu_ref_tryget[_live]() are safe to perform on @ref while
* this function is in progress.
@@ -357,10 +349,11 @@ EXPORT_SYMBOL_GPL(percpu_ref_kill_and_confirm);
void percpu_ref_reinit(struct percpu_ref *ref)
{
unsigned long flags;
+ unsigned long __percpu *percpu_count;

spin_lock_irqsave(&percpu_ref_switch_lock, flags);

- WARN_ON_ONCE(!percpu_ref_is_zero(ref));
+ WARN_ON_ONCE(__ref_is_percpu(ref, &percpu_count));

ref->percpu_count_ptr &= ~__PERCPU_REF_DEAD;
percpu_ref_get(ref);
--
2.9.5