cgroup: make sure that decisions in __css_put are atomic

__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@google.com>
Acked-by: Li Zefan <lizefan@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 72fcd3069a909064529f1eb730e4985d16cef006..ceeafe874b3fda0e51cef70d20b9e809f094fc77 100644 (file)
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -4984,8 +4984,7 @@ 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);