linux-kernel - Re: [PATCH v3] mm, oom: Fix race when selecting process to kill

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Message-ID: <20131109151639.GB14249@redhat.com>
Date:	Sat, 9 Nov 2013 16:16:39 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	Sameer Nanda <snanda@...omium.org>
Cc:	akpm@...ux-foundation.org, mhocko@...e.cz, rientjes@...gle.com,
	hannes@...xchg.org, rusty@...tcorp.com.au, semenzato@...gle.com,
	murzin.v@...il.com, dserrg@...il.com, msb@...omium.org,
	linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3] mm, oom: Fix race when selecting process to kill

On 11/08, Sameer Nanda wrote:
>
> @@ -413,12 +413,20 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
>  					      DEFAULT_RATELIMIT_BURST);
> @@ -456,10 +463,18 @@ void oom_kill_process(struct task_struct *p, gfp_t gfp_mask, int order,
>  			}
>  		}
>  	} while_each_thread(p, t);
> -	read_unlock(&tasklist_lock);
>  
>  	rcu_read_lock();
> +
>  	p = find_lock_task_mm(victim);
> +
> +	/*
> +	 * Since while_each_thread is currently not RCU safe, this unlock of
> +	 * tasklist_lock may need to be moved further down if any additional
> +	 * while_each_thread loops get added to this function.
> +	 */
> +	read_unlock(&tasklist_lock);

Well, ack... but with this change find_lock_task_mm() relies on tasklist,
so it makes sense to move rcu_read_lock() down before for_each_process().
Otherwise this looks confusing, but I won't insist.

Oleg.

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