linux-kernel - Re: [linux-pm] Re: Hibernation considerations

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Message-Id: <200707240755.01820.nigel@nigel.suspend2.net>
Date:	Tue, 24 Jul 2007 07:55:00 +1000
From:	Nigel Cunningham <nigel@...el.suspend2.net>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	nigel@...pend2.net, Paul Mackerras <paulus@...ba.org>,
	david@...g.hm, Miklos Szeredi <miklos@...redi.hu>,
	linux-kernel@...r.kernel.org, miltonm@....com,
	ying.huang@...el.com, linux-pm@...ts.linux-foundation.org,
	Jeremy Maitin-Shepard <jbms@....edu>
Subject: Re: [linux-pm] Re: Hibernation considerations

Hi.

On Tuesday 24 July 2007 01:23:15 Alan Stern wrote:
> On Mon, 23 Jul 2007, Nigel Cunningham wrote:
> 
> > Take a step back for a second.
> > 
> > The problem we're facing now is that we're getting some userspace threads, 
> > used in processing I/O, that are functioning as exceptions to the "freeze 
> > userspace, then freezeable kernel threads" rule. They are only exceptions 
> > because of that role in processing I/O - because they're de facto kernel 
> > threads. So, if we orient our thinking more in terms of I/O processing and 
> > less in terms of the userspace/kernelspace distinction, we'll have a 
> > solution:
> > 
> > 1) Freeze processes that aren't fs related (ie stop them generating I/O).
> 
> The problem here is that with things like FUSE, _every_ process is 
> potentially fs related.  Nothing prevents a FUSE thread from doing IPC 
> with any other thread.

Yes, but the fuse thread is going to know what other thread it's doing IPC 
with, so it can get that thread flagged too.

> > 2) Flush pending I/O.
> > 3) Freeze filesystems in reverse order of dependency, the primary purpose 
> > being to stop them generating further I/O on their metadata.
> > 
> > Locks that are being held are only being held because work is being done. 
If 
> > we progressively focus on threads in terms of their create/process work 
> > dependencies, we'll see that the problem isn't at all intractable.
> 
> As has been mentioned before, keeping track of all that dependency 
> information would be very fragile and time-consuming.

I disagree. It's at least going to be less fragile and time-consuming then 
maintaining new/extra code for kexec.

Nigel


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