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

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Date:	Mon, 23 Jul 2007 11:23:15 -0400 (EDT)
From:	Alan Stern <stern@...land.harvard.edu>
To:	nigel@...pend2.net
cc:	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

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.

> 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.

Alan Stern

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