__css_put is using atomic_dec on the ref count, and then
looking at the ref count to make decisions. This is prone
to races, as someone else may decrement ref count between
our decrement and our decision. Instead, we should base our
decisions on the value that we decremented the ref count to.
(This results in an actual race on Google's kernel which I
haven't been able to reproduce on the upstream kernel. Having
said that, it's still incorrect by inspection).
Signed-off-by: Salman Qazi <sqazi@xxxxxxxxxx>
---
kernel/cgroup.c | 3 +--
1 files changed, 1 insertions(+), 2 deletions(-)
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 0f3527d..18dc8aa 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4973,8 +4973,7 @@ void __css_put(struct cgroup_subsys_state *css)
struct cgroup *cgrp = css->cgroup;
rcu_read_lock();
- atomic_dec(&css->refcnt);
- switch (css_refcnt(css)) {
+ switch (atomic_dec_return(&css->refcnt)) {
case 1:
if (notify_on_release(cgrp)) {
set_bit(CGRP_RELEASABLE, &cgrp->flags);
--
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