linux-kernel - Re: [PATCH] mm: Use rcu_dereference in in

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Date:   Mon, 22 Feb 2021 09:30:27 +0100
From:   Michal Hocko <mhocko@...e.com>
To:     "Matthew Wilcox (Oracle)" <willy@...radead.org>
Cc:     linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Use rcu_dereference in in_vfork

On Sun 21-02-21 19:42:06, Matthew Wilcox wrote:
> Fix a sparse warning by using rcu_dereference().  Technically this is a
> bug and a sufficiently aggressive compiler could reload the `real_parent'
> pointer outside the protection of the rcu lock (and access freed memory),
> but I think it's pretty unlikely to happen.
> 
> Fixes: b18dc5f291c0 ("mm, oom: skip vforked tasks from being selected")
> Signed-off-by: Matthew Wilcox (Oracle) <willy@...radead.org>

I must have missed the RCU part.
Thanks!

Acked-by: Michal Hocko <mhocko@...e.com>

> ---
>  include/linux/sched/mm.h | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/sched/mm.h b/include/linux/sched/mm.h
> index 1ae08b8462a4..90b2a0bce11c 100644
> --- a/include/linux/sched/mm.h
> +++ b/include/linux/sched/mm.h
> @@ -140,7 +140,8 @@ static inline bool in_vfork(struct task_struct *tsk)
>  	 * another oom-unkillable task does this it should blame itself.
>  	 */
>  	rcu_read_lock();
> -	ret = tsk->vfork_done && tsk->real_parent->mm == tsk->mm;
> +	ret = tsk->vfork_done &&
> +			rcu_dereference(tsk->real_parent)->mm == tsk->mm;
>  	rcu_read_unlock();
>  
>  	return ret;
> -- 
> 2.29.2
> 

-- 
Michal Hocko
SUSE Labs