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

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Date:	Sat, 12 May 2007 02:01:41 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Gautham R Shenoy <ego@...ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Oleg Nesterov <oleg@...sign.ru>, 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 Saturday, 12 May 2007 01:29, Linus Torvalds wrote:
> 
> On Sat, 12 May 2007, Rafael J. Wysocki wrote:
> >
> > We use this function (ie. kernel/power/process.c:is_user_space()) to
> > distinguish kernel threads from user space processes.  Therefore we make it
> > always return true for user space processes and always return false for kernel
> > threads.  In the latter case we need to use the task_lock() to ensure that the
> > result is as desired (ie. false), because otherwise it might be racing with
> > either fs/aio.c:use_mm() or fs/aio.c:unuse_mm().
> 
> But there is no race protection in the *caller*, so if it can ever return 
> one or the other, what protects it from changing once the caller returns?
> 
> And if the value can change (because some thread uses "use_mm()"), then 
> the caller cannot rely on the value that got returned.

The value cannot change because of that.  There only is a small window inside
unuse_mm() (or use_mm()) in which the value may be wrong.  Namely:

static void unuse_mm(struct mm_struct *mm)
{
	struct task_struct *tsk = current;

	task_lock(tsk);
	tsk->flags &= ~PF_BORROWED_MM;
---
--- If is_user_space() without the task_lock() is called right here, it will
--- return 'true', although it should return 'false'.
---
	tsk->mm = NULL;
	/* active_mm is still 'mm' */
	enter_lazy_tlb(mm, tsk);
	task_unlock(tsk);
}

IOW, quoting Andrew, "is_user_space() requires that the state of p->mm and
p->flags be consistent".

> So you migt as well not return any value at all, since the returned value 
> is apparently meaningless once the lock has been released.

No, it is not meaningless.

Rafael
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