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Subject[GIT PULL] cgroup fixes for v3.5-rc3
Hello, Linus.

Please pull from the following branch to receive two patches fixing a
refcnt race bug during css_put(). Decing and checking the value
weren't atomic and two tasks could think that they both pushed the
counter to zero.

git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup.git for-3.5-fixes

Salman Qazi (2):
cgroup: make sure that decisions in __css_put are atomic
cgroups: Account for CSS_DEACT_BIAS in __css_put

Thanks.
---
kernel/cgroup.c | 13 ++++++++++---
1 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 72fcd30..2097684 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -255,12 +255,17 @@ int cgroup_lock_is_held(void)

EXPORT_SYMBOL_GPL(cgroup_lock_is_held);

+static int css_unbias_refcnt(int refcnt)
+{
+ return refcnt >= 0 ? refcnt : refcnt - CSS_DEACT_BIAS;
+}
+
/* the current nr of refs, always >= 0 whether @css is deactivated or not */
static int css_refcnt(struct cgroup_subsys_state *css)
{
int v = atomic_read(&css->refcnt);

- return v >= 0 ? v : v - CSS_DEACT_BIAS;
+ return css_unbias_refcnt(v);
}

/* convenient tests for these bits */
@@ -4982,10 +4987,12 @@ EXPORT_SYMBOL_GPL(__css_tryget);
void __css_put(struct cgroup_subsys_state *css)
{
struct cgroup *cgrp = css->cgroup;
+ int v;

rcu_read_lock();
- atomic_dec(&css->refcnt);
- switch (css_refcnt(css)) {
+ v = css_unbias_refcnt(atomic_dec_return(&css->refcnt));
+
+ switch (v) {
case 1:
if (notify_on_release(cgrp)) {
set_bit(CGRP_RELEASABLE, &cgrp->flags);

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