[3.13.y.z extended stable] Patch "percpu-refcount: fix usage of this_cpu_ops" has been added to staging queue

[Kamal Mostafa]

This is a note to let you know that I have just added a patch titled

    percpu-refcount: fix usage of this_cpu_ops

to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree 
which can be found at:

 http://kernel.ubuntu.com/git?p=ubuntu/linux.git;a=shortlog;h=refs/heads/linux-3.13.y-queue

This patch is scheduled to be released in version 3.13.11.4.

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y.z tree, see
https://wiki.ubuntu.com/Kernel/Dev/ExtendedStable

Thanks.
-Kamal

------

>From d103b9d54740cc7b2dfaa50af70ff6f02ed97b7e Mon Sep 17 00:00:00 2001
From: Sebastian Ott <sebott at linux.vnet.ibm.com>
Date: Wed, 4 Jun 2014 15:58:24 +0200
Subject: percpu-refcount: fix usage of this_cpu_ops

commit 0c36b390a546055b6815d4b93a2c9fed4d980ffb upstream.

The percpu-refcount infrastructure uses the underscore variants of
this_cpu_ops in order to modify percpu reference counters.
(e.g. __this_cpu_inc()).

However the underscore variants do not atomically update the percpu
variable, instead they may be implemented using read-modify-write
semantics (more than one instruction).  Therefore it is only safe to
use the underscore variant if the context is always the same (process,
softirq, or hardirq). Otherwise it is possible to lose updates.

This problem is something that Sebastian has seen within the aio
subsystem which uses percpu refcounters both in process and softirq
context leading to reference counts that never dropped to zeroes; even
though the number of "get" and "put" calls matched.

Fix this by using the non-underscore this_cpu_ops variant which
provides correct per cpu atomic semantics and fixes the corrupted
reference counts.

Cc: Kent Overstreet <kmo at daterainc.com>
Reported-by: Sebastian Ott <sebott at linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens at de.ibm.com>
Signed-off-by: Tejun Heo <tj at kernel.org>
References: http://lkml.kernel.org/g/alpine.LFD.2.11.1406041540520.21183@denkbrett
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
---
 include/linux/percpu-refcount.h | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
index 95961f0..0afb48f 100644
--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -110,7 +110,7 @@ static inline void percpu_ref_get(struct percpu_ref *ref)
 	pcpu_count = ACCESS_ONCE(ref->pcpu_count);

 	if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR))
-		__this_cpu_inc(*pcpu_count);
+		this_cpu_inc(*pcpu_count);
 	else
 		atomic_inc(&ref->count);

@@ -139,7 +139,7 @@ static inline bool percpu_ref_tryget(struct percpu_ref *ref)
 	pcpu_count = ACCESS_ONCE(ref->pcpu_count);

 	if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR)) {
-		__this_cpu_inc(*pcpu_count);
+		this_cpu_inc(*pcpu_count);
 		ret = true;
 	}

@@ -164,7 +164,7 @@ static inline void percpu_ref_put(struct percpu_ref *ref)
 	pcpu_count = ACCESS_ONCE(ref->pcpu_count);

 	if (likely(REF_STATUS(pcpu_count) == PCPU_REF_PTR))
-		__this_cpu_dec(*pcpu_count);
+		this_cpu_dec(*pcpu_count);
 	else if (unlikely(atomic_dec_and_test(&ref->count)))
 		ref->release(ref);

--
1.9.1