[GIT PULL] percpu fixes for v3.18-rc5

From: Tejun Heo
Date: Sun Nov 23 2014 - 12:48:16 EST

Hello, Linus.

This pull request contains one patch to fix a race condition which can
lead to percpu_ref using a percpu pointer which is corrupted with a
set DEAD bit. The bug was introduced while separating out the ATOMIC
mode flag from the DEAD flag. The fix is pretty straight forward.

I just committed the patch to the percpu tree but am sending out the
pull request early as I'll be on vacation for a week. The patch
should be fairly safe and while the latency will be higher I'll be
checking emails.


The following changes since commit cac7f2429872d3733dc3f9915857b1691da2eb2f:

Linux 3.18-rc2 (2014-10-26 16:48:41 -0700)

are available in the git repository at:

git://git.kernel.org/pub/scm/linux/kernel/git/tj/percpu.git for-3.18-fixes

for you to fetch changes up to 4aab3b5b3ccf94fc907e66233e6ca4d8675759a6:

percpu-ref: fix DEAD flag contamination of percpu pointer (2014-11-23 12:36:06 -0500)

Tejun Heo (1):
percpu-ref: fix DEAD flag contamination of percpu pointer

include/linux/percpu-refcount.h | 8 +++++++-
1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/include/linux/percpu-refcount.h b/include/linux/percpu-refcount.h
index d5c89e0..51ce60c 100644
--- a/include/linux/percpu-refcount.h
+++ b/include/linux/percpu-refcount.h
@@ -133,7 +133,13 @@ static inline bool __ref_is_percpu(struct percpu_ref *ref,
/* paired with smp_store_release() in percpu_ref_reinit() */

- if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC))
+ /*
+ * Theoretically, the following could test just ATOMIC; however,
+ * then we'd have to mask off DEAD separately as DEAD may be
+ * visible without ATOMIC if we race with percpu_ref_kill(). DEAD
+ * implies ATOMIC anyway. Test them together.
+ */
+ if (unlikely(percpu_ptr & __PERCPU_REF_ATOMIC_DEAD))
return false;

*percpu_countp = (unsigned long __percpu *)percpu_ptr;
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