linux-kernel - Re: [PATCH 1/7] Freezer: Read PF_BORROWED

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Message-ID: <20070512141813.GA98@tv-sign.ru>
Date:	Sat, 12 May 2007 18:18:13 +0400
From:	Oleg Nesterov <oleg@...sign.ru>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Gautham R Shenoy <ego@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>, Pavel Machek <pavel@....cz>,
	"Eric W. Biederman" <ebiederm@...ssion.com>
Subject: Re: [PATCH 1/7] Freezer: Read PF_BORROWED_MM in a nonracy way

On 05/12, Rafael J. Wysocki wrote:
>
> ... user space tasks that call deamonize() can also be frozen prematurely.
> We didn't take this possibility into consideration before, which was obviously
> wrong.

No, no, sorry for the confusion. User space tasks never call deamonize().

Kernel threads call daemonize, because when we are doing kernel_thread()
on behalf of user-space task, the new kernel thread (child) shares its
->mm with the caller (parent). So it is considered as "is_user_space()"
until it does daemonize().

Definitely, is_user_space() should have another name.

When a user space task exits, it does exit_mm() and becomes "a kernel thread"
from the freezer POV. In its current from, freezer can do nothing with this.
The exiting task won't call try_to_freeze() after that, so try_to_freeze_tasks()
will wait until it dissapears (actually, until it calls exit_notify(), note
the ->exit_state check in freezeable()).

I do not think we can improve things if exit_mm() clears TIF_FREEZING.
We should clear TIF_FREEZING when we set PF_NOFREEZE, I think. This was
discussed before iirc, but I forgot the result.

Oleg.

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