[PATCH percpu/for-3.11] percpu-refcount: consistently use plain (non-sched) RCU

[PATCH percpu/for-3.11] percpu-refcount: consistently use plain (non-sched) RCU

[Tejun Heo]

>From e96262150a513ce3d54ff221d4ace8aeec96e0bf Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Wed, 12 Jun 2013 13:37:42 -0700

percpu_ref_get/put() are using preempt_disable/enable() while
percpu_ref_kill() is using plain call_rcu() instead of
call_rcu_sched().  This is buggy as grace periods of the two may not
match.  Fix it by using plain RCU in percpu_ref_get/put().

(I suggested using sched RCU in the first place but there's no actual
 benefit in doing so unless we're gonna introduce different variants
 of get/put to be called while preemption is alredy disabled, which we
 definitely shouldn't.)

Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: Rusty Russell <rusty@rustcorp.com.au>
---
 include/linux/percpu-refcount.h | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
index 24b31ef..abe1411 100644
--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -85,7 +85,7 @@ static inline void percpu_ref_get(struct percpu_ref *ref)
 {
 	unsigned __percpu *pcpu_count;
 
-	preempt_disable();
+	rcu_read_lock();
 
 	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
 
@@ -94,7 +94,7 @@ static inline void percpu_ref_get(struct percpu_ref *ref)
 	else
 		atomic_inc(&ref->count);
 
-	preempt_enable();
+	rcu_read_unlock();
 }
 
 /**
@@ -107,7 +107,7 @@ static inline void percpu_ref_put(struct percpu_ref *ref)
 {
 	unsigned __percpu *pcpu_count;
 
-	preempt_disable();
+	rcu_read_lock();
 
 	pcpu_count = ACCESS_ONCE(ref->pcpu_count);
 
@@ -116,7 +116,7 @@ static inline void percpu_ref_put(struct percpu_ref *ref)
 	else if (unlikely(atomic_dec_and_test(&ref->count)))
 		ref->release(ref);
 
-	preempt_enable();
+	rcu_read_unlock();
 }
 
 #endif
-- 
1.8.2.1

Re: [PATCH percpu/for-3.11] percpu-refcount: consistently use plain (non-sched) RCU

[Kent Overstreet]

On Wed, Jun 12, 2013 at 01:40:32PM -0700, Tejun Heo wrote:
> From e96262150a513ce3d54ff221d4ace8aeec96e0bf Mon Sep 17 00:00:00 2001
> From: Tejun Heo <tj@kernel.org>
> Date: Wed, 12 Jun 2013 13:37:42 -0700
> 
> percpu_ref_get/put() are using preempt_disable/enable() while
> percpu_ref_kill() is using plain call_rcu() instead of
> call_rcu_sched().  This is buggy as grace periods of the two may not
> match.  Fix it by using plain RCU in percpu_ref_get/put().
> 
> (I suggested using sched RCU in the first place but there's no actual
>  benefit in doing so unless we're gonna introduce different variants
>  of get/put to be called while preemption is alredy disabled, which we
>  definitely shouldn't.)
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Reported-by: Rusty Russell <rusty@rustcorp.com.au>

Acked-by: Kent Overstreet <koverstreet@google.com>

Re: [PATCH percpu/for-3.11] percpu-refcount: consistently use plain (non-sched) RCU

[Tejun Heo]

On Wed, Jun 12, 2013 at 01:40:32PM -0700, Tejun Heo wrote:
> From e96262150a513ce3d54ff221d4ace8aeec96e0bf Mon Sep 17 00:00:00 2001
> From: Tejun Heo <tj@kernel.org>
> Date: Wed, 12 Jun 2013 13:37:42 -0700
> 
> percpu_ref_get/put() are using preempt_disable/enable() while
> percpu_ref_kill() is using plain call_rcu() instead of
> call_rcu_sched().  This is buggy as grace periods of the two may not
> match.  Fix it by using plain RCU in percpu_ref_get/put().
> 
> (I suggested using sched RCU in the first place but there's no actual
>  benefit in doing so unless we're gonna introduce different variants
>  of get/put to be called while preemption is alredy disabled, which we
>  definitely shouldn't.)
> 
> Signed-off-by: Tejun Heo <tj@kernel.org>
> Reported-by: Rusty Russell <rusty@rustcorp.com.au>

Applied to percpu/for-3.11.

Thanks.

-- 
tejun