Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751900AbaFDQhO (ORCPT ); Wed, 4 Jun 2014 12:37:14 -0400 Received: from mail-qa0-f47.google.com ([209.85.216.47]:44884 "EHLO mail-qa0-f47.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751049AbaFDQhN (ORCPT ); Wed, 4 Jun 2014 12:37:13 -0400 Date: Wed, 4 Jun 2014 12:37:00 -0400 From: Tejun Heo To: Linus Torvalds Cc: linux-kernel@vger.kernel.org, Kent Overstreet , Sebastian Ott , Heiko Carstens , Christoph Lameter Subject: [GIT PULL] last minute percpu fix for v3.15-rc8 Message-ID: <20140604163700.GH5004@htj.dyndns.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.23 (2014-03-12) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, Linus. It is very late but this is an important percpu-refcount fix from Sebastian Ott. The problem is that percpu_ref_*() used __this_cpu_*() instead of this_cpu_*(). The difference between the two is that the latter is atomic on the local cpu while the former is not. this_cpu_inc() is guaranteed to increment the percpu counter on the cpu that the operation is executed on without any synchronization; however, __this_cpu_inc() doesn't and if the local cpu invokes the function from different contexts (e.g. process and irq) of the same CPU, it's not guaranteed to actually increment as it may be implemented as rmw. This bug existed from the get-go but it hasn't been noticed earlier probably because on x86 __this_cpu_inc() is equivalent to this_cpu_inc() as both get translated into single instruction; however, s390 uses the generic rmw implementation and gets affected by the bug. Kudos to Sebastian and Heiko for diagnosing it. The change is very low risk and fixes a critical issue on the affected architectures, so I think it's a good candidate for inclusion although it's very late in the devel cycle. On the other hand, this has been broken since v3.11, so backporting it through -stable post -rc1 won't be the end of the world. I'll ping Christoph whether __this_cpu_*() ops can be better annotated so that it can trigger lockdep warning when used from multiple contexts. Thanks. The following changes since commit 5a838c3b60e3a36ade764cf7751b8f17d7c9c2da: percpu: make pcpu_alloc_chunk() use pcpu_mem_free() instead of kfree() (2014-04-14 16:18:06 -0400) are available in the git repository at: git://git.kernel.org/pub/scm/linux/kernel/git/tj/percpu.git for-3.15-fixes for you to fetch changes up to 0c36b390a546055b6815d4b93a2c9fed4d980ffb: percpu-refcount: fix usage of this_cpu_ops (2014-06-04 12:12:29 -0400) ---------------------------------------------------------------- Sebastian Ott (1): percpu-refcount: fix usage of this_cpu_ops 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); -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/