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Subject: Re: [PATCH] mm: Use rcu_dereference in in_vfork
To:     "Matthew Wilcox (Oracle)" <willy@infradead.org>
CC:     <linux-kernel@vger.kernel.org>, Michal Hocko <mhocko@suse.com>,
        <linux-mm@kvack.org>, Andrew Morton <akpm@linux-foundation.org>
References: <20210221194207.1351703-1-willy@infradead.org>
From:   Miaohe Lin <linmiaohe@huawei.com>
Message-ID: <28850db2-687d-e604-2bd3-3a72344c1e7d@huawei.com>
Date:   Mon, 22 Feb 2021 09:44:39 +0800
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On 2021/2/22 3:42, Matthew Wilcox (Oracle) 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@infradead.org>

Looks good to me. Thanks!
Reviewed-by: Miaohe Lin <linmiaohe@huawei.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;
>