Message-Id: <200707060935.12706.nigel@nigel.suspend2.net>
Date:	Fri, 6 Jul 2007 09:35:11 +1000
From:	Nigel Cunningham <nigel@...el.suspend2.net>
To:	Benjamin Herrenschmidt <benh@...nel.crashing.org>
Cc:	Pavel Machek <pavel@....cz>, "Rafael J. Wysocki" <rjw@...k.pl>,
	Matthew Garrett <mjg59@...f.ucam.org>,
	linux-kernel@...r.kernel.org, linux-pm@...ts.linux-foundation.org,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: [PATCH] Remove process freezer from suspend to RAM pathway

Hi.

On Friday 06 July 2007 09:20:43 Benjamin Herrenschmidt wrote:
> 
> > Will you be able to guarantee that every place where a task can/will block 
> > will be harmless place? If so, how will you guarantee that? How will you 
> > debug issues where a task occasionally doesn't block in the right place, 
> > particularly instances where it is some less than obvious interaction with 
> > other tasks?
> 
> Which places aren't harmless if you don't have a freezer ?

If I knew that, I wouldn't be asking the question.

> > This is the whole point to having the freezer. It makes things more 
> > predictable and testable. It shows us, clearly, when process X is the one 
> > that is causing problems.
> 
> No, the freezer creates all those places what are harmful for a task to
> block because they will break the freezer :-)

Nice try :) Okay then, you remove the freezer, try hibernating, then get back 
to me after you've fixed your filesystem because some process that wasn't 
frozen started writing things after the atomic copy (making the on disk 
filesystem inconsistent with the snapshot).

As Pavel rightly said, you can get rid of the freezer, but you're only going 
to have to implement another one that does the essentially the same thing, 
even if it is at some other level.
 
> > >  - Silently add GFP_NOIO to all allocations, to avoid having things
> > > blocking in kmalloc() with a mutex held that will deadlock with
> > > suspend() in a driver for example. Or set some way to have all GFP
> > > waiters wakeup and fail rather than wait for IOs. It's hard/bizarre but
> > > necessary, again, with or without a freezer.
> > 
> > GFP_ATOMIC? (In driver suspend, they shouldn't be sleeping either, right?)
> 
> NOIO should be enough I think but ATOMIC would do).
>  
> That's one of the reason why I used to have the pre-suspend and
> post-resume hooks in my original powermac implementation, for those few
> drivers complicated enough to require some pre-allocations.
>  
> > >  - Deal with the firmware problem. The best way is probably to have an
> > > async request_firmware interface(). Another thing is, drivers may want
> > > to cache their firmware in main memory, that sort of thing...
> > >
> 
> Note that the above firmware problem could be dealt with also with the
> pre-suspend/post-resume. Allowing to pre-request firmware etc... and
> keep it around until after resume, because we know we will need it.
> Gives a chance to drivers to perform things while the system is still
> live, filesystems still working, etc... (big memory allocations for
> example).
> 
> > > And that's just a small list off the top of my mind, of known problems
> > > that will cause deadlocks or misbehaviours today, with or without the
> > > freezer, and that need to be addressed.
> > 
> > Userspace device drivers too?
> 
> Maybe but they are less of an issue, most of the time, they don't do DMA
> or whatever harmful things. If they are USB drivers, for example, they
> are an non-issues at that level.

(Leaving the rest of the message intact so we don't have to fragment the 
discussion into a million subthreads).

Regards,

Nigel

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