linux-kernel - Re: [PATCH] mm : Avoid candidate task is equal to current task

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Message-ID: <20140416135741.GA9407@redhat.com>
Date:	Wed, 16 Apr 2014 15:57:42 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Chiang <pchiang@...dia.com>
Cc:	"ccross@...roid.com" <ccross@...roid.com>,
	"lizefan@...wei.com" <lizefan@...wei.com>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"tj@...nel.org" <tj@...nel.org>, "pavel@....cz" <pavel@....cz>,
	"ebiederm@...ssion.com" <ebiederm@...ssion.com>,
	"guillaume@...infr.org" <guillaume@...infr.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm : Avoid candidate task is equal to current task

On 04/16, Peter Chiang wrote:
>
> mm_update_next_owner() call from exit_mm() , and exit_mm()  change tsk->mm
> to NULL If p==c , It seems to be  impossible that mm == c->mm  (tsk->mm) .
> Because mm is non-NULL and  c->mm is NULL .

Confused, please see below.

> Fix kernel panic when finding a new owner for the mm and the new owner is
> equal to current onwer

Did you actually observe the panic ?

> diff --git a/kernel/exit.c b/kernel/exit.c index 6ed6a1d..aa98422 100644
> --- a/kernel/exit.c
> +++ b/kernel/exit.c
> @@ -399,7 +399,7 @@ retry:
>  	 * here often
>  	 */
>  	do_each_thread(g, c) {
> -		if (c->mm == mm)
> +		if ((c != p) && (c->mm == mm))
>  			goto assign_new_owner;
>  	} while_each_thread(g, c);

p == current. This is always called with p->mm == NULL and mm != NULL.

So, if c->mm == mm then at least c->mm != NULL, and this means that
c == p is not possible?

And it seems that this is exactly what you meant above. So why do you
think we need this change?

Oleg.

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