linux-kernel - Re: [linux-pm] Re: [PATCH] Remove process freezer from suspend to RAM pathway

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Message-Id: <200707060920.43707.rjw@sisk.pl>
Date:	Fri, 6 Jul 2007 09:20:42 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Alan Stern <stern@...land.harvard.edu>,
	Paul Mackerras <paulus@...ba.org>,
	Johannes Berg <johannes@...solutions.net>,
	Linux-pm mailing list <linux-pm@...ts.linux-foundation.org>,
	Kernel development list <linux-kernel@...r.kernel.org>,
	Pavel Machek <pavel@....cz>,
	Matthew Garrett <mjg59@...f.ucam.org>
Subject: Re: [linux-pm] Re: [PATCH] Remove process freezer from suspend to RAM pathway

On Friday, 6 July 2007 00:59, Benjamin Herrenschmidt wrote:
> On Thu, 2007-07-05 at 10:23 -0400, Alan Stern wrote:
> > 
> > How will that help?  Block the kernel thread in the freezer or block it 
> > in the driver -- either way it is blocked.  So how do your deadlocks 
> > get resolved?
> 
> Because nobody is waiting on that kernel thread anyway without a freezer
> so there is no deadlock anymore.

I'm not sure what you mean.  The freezer doesn't wait for threads that are
already frozen ...

> > I disagree with your analysis -- not that it's completely wrong, but it 
> > points out an existing basic problem in the kernel.  The kernel should 
> > never depend on userspace!  More correctly, a task executing in the 
> > kernel should never block with any sort of mutex or other lock held (in 
> > a way that would preclude it from being frozen, let's say) while 
> > waiting for a response from userspace.
> > 
> > Then the dependency graph would be easy to construct: User tasks can
> > depend on whatever they want, and kernel threads never depend on a user
> > task.
> 
> In an idea world, there would be no hunger...
> 
> > If this contradicts the existing implementations and APIs for userspace 
> > filesystems, then so be it.  My conclusion would be that the 
> > implementations and APIs should be changed.
> 
> Why are you guys working so hard and spending so much energy to try to
> avoid doing the right thing is beyond my understanding...
> 
> > It _does_ apply to kernel threads.  That's exactly why I wrote above 
> > that kernel threads which try to do I/O during a suspend will need 
> > extra attention.
> 
> Ok none at all if you don't have a freezer.

Provided that drivers can handle that.

Greetings,
Rafael


-- 
"Premature optimization is the root of all evil." - Donald Knuth
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