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

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Message-Id: <200705121245.50715.rjw@sisk.pl>
Date:	Sat, 12 May 2007 12:45:49 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Oleg Nesterov <oleg@...sign.ru>,
	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 Saturday, 12 May 2007 11:01, Rafael J. Wysocki wrote:
> On Saturday, 12 May 2007 03:24, Linus Torvalds wrote:
> > 
> > On Sat, 12 May 2007, Oleg Nesterov wrote:
> > > 
> > > However, in my opininon THAT PATCH has nothing to do with this problem.
> > > It just improves the code that we already have.
> > 
> > Sure. 
> > 
> > However, I think it does it THE WRONG WAY, and doesn't actually fix the 
> > much deeper problems with the freezer, as shown by the fact that the lock 
> > is *still* broken for other cases.
> 
> The other cases don't lead to the specific issue this patch is meant to
> prevent.  Namely, that if a kernel thread is identified as a user space task by
> the freezer it may be frozen prematurely.
> 
> And yes, this only is a problem because we freeze kernel threads, which may be
> avoidable, but we've been doing that for more than two years and it's a big
> change to stop doing so.  We can't just say overnight that we won't be freezing
> kernel threads from now on without al least checking if that doesn't lead to
> user-visible problems.
> 
> Thus IMO it's reasonable to fix the potential issue with the current code and
> think about changing the design *later*.  Still, I'm not so attached to this
> patch and if you think that it should be dropped (which IMO is wrong), then so
> be it.

Sorry, I was wrong, because ...

> That said:
> 
> > So, here's a summary:
> > 
> >  - we should not take the lock inside the function, because taking it 
> >    there is fundamentally wrong, and leaves all the *other* races in 
> >    place.
> 
> The other races don't lead to the same (wrong) result.
> 
> >  - if you actually want to solve the other races, the lock needs to be 
> >    taken by the caller, in which case taking it in the callee is obviously 
> >    (again) wrong.
> > 
> >  - or then, we accept that the race wasn't fixed AT ALL, and you add other 
> >    code to _other_ places to handle the case where you froze the wrong 
> >    thread (or didn't freeze the right one).
> 
> There are no other cases, AFAICS, in which we can freeze a wrong thead.

... user space tasks that call deamonize() can also be frozen prematurely.
We didn't take this possibility into consideration before, which was obviously
wrong.

Rafael
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