Re: [PATCH linux-next] cgroup: fix suspicious rcu_dereference_check() usage warning

[Marek Szyprowski]

On 05.03.2022 04:41, Chengming Zhou wrote:
> task_css_set_check() will use rcu_dereference_check() to check for
> rcu_read_lock_held() on the read-side, which is not true after commit
> dc6e0818bc9a ("sched/cpuacct: Optimize away RCU read lock"). This
> commit drop explicit rcu_read_lock(), change to RCU-sched read-side
> critical section. So fix the RCU warning by adding check for
> rcu_read_lock_sched_held().
>
> Fixes: dc6e0818bc9a ("sched/cpuacct: Optimize away RCU read lock")
> Reported-by: Linux Kernel Functional Testing <lkft@xxxxxxxxxx>
> Reported-by: syzbot+16e3f2c77e7c5a0113f9@xxxxxxxxxxxxxxxxxxxxxxxxx
> Tested-by: Zhouyi Zhou <zhouzhouyi@xxxxxxxxx>
> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>

Right, this fixes the issue I've reported here:

https://lore.kernel.org/all/f4bc652b-115f-35b5-91db-bad3b30fed9b@xxxxxxxxxxx/

Tested-by: Marek Szyprowski <m.szyprowski@xxxxxxxxxxx>

> ---
>   include/linux/cgroup.h | 1 +
>   1 file changed, 1 insertion(+)
>
> diff --git a/include/linux/cgroup.h b/include/linux/cgroup.h
> index 1e356c222756..0d1ada8968d7 100644
> --- a/include/linux/cgroup.h
> +++ b/include/linux/cgroup.h
> @@ -450,6 +450,7 @@ extern struct mutex cgroup_mutex;
>   extern spinlock_t css_set_lock;
>   #define task_css_set_check(task, __c)					\
>   	rcu_dereference_check((task)->cgroups,				\
> +		rcu_read_lock_sched_held() ||				\
>   		lockdep_is_held(&cgroup_mutex) ||			\
>   		lockdep_is_held(&css_set_lock) ||			\
>   		((task)->flags & PF_EXITING) || (__c))

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland