linux-kernel - Re: [RFC][PATCH -mm 3/3] PM: Disable _request

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Message-ID: <20070527220158.GB22687@srcf.ucam.org>
Date:	Sun, 27 May 2007 23:01:58 +0100
From:	Matthew Garrett <mjg59@...f.ucam.org>
To:	"Rafael J. Wysocki" <rjw@...k.pl>
Cc:	pm list <linux-pm@...ts.linux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Nigel Cunningham <nigel@...el.suspend2.net>,
	Pavel Machek <pavel@....cz>,
	Alan Stern <stern@...land.harvard.edu>,
	Oliver Neukum <oliver@...kum.org>
Subject: Re: [RFC][PATCH -mm 3/3] PM: Disable _request_firmware before hibernation/suspend

On Sun, May 27, 2007 at 11:45:03PM +0200, Rafael J. Wysocki wrote:
> On Sunday, 27 May 2007 22:49, Matthew Garrett wrote:
> > If we remove process freezing in STR, this should just work[1] without the
> > need to complicate things. 
> 
> Under the (optimistic, IMO) assumption that the relevant user space task won't
> block on I/O with a suspended device involved or something like this.

Yeah, I forgot the footnote that was going to mention that. Clearly 
there are issues if this is on some block device that hasn't been 
resumed yet.

> BTW, I know of two subsystems that want their kernel threads to be frozen for
> synchronization purposes.  Please see these messages:
> 
> 1) https://lists.linux-foundation.org/pipermail/linux-pm/2007-May/012592.html
> (plus follow up)
> 
> 2) http://marc.info/?l=linux-kernel&m=117919066830575&w=2

I'm not entirely sold on this. The issue is that there's the possibility 
of races during suspend/resume? It sounds like that should be 
implemented in the driver, rather than depending on a side-effect of 
process freezing. Otherwise there's no way of selectively suspending 
that device.

> Besides, there's the hibernation that needs to freeze tasks for another reason,
> so it needs some way to ensure that drivers won't request firmware while
> the user land is frozen.

I agree that that's an issue right now, but I think we should see if 
this is repairable without just breaking those drivers. One option would 
be to defer resuming them until userspace is alive again - that would be 
no worse than the suspend to RAM case without process freezing.

-- 
Matthew Garrett | mjg59@...f.ucam.org
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