Date:	Wed, 25 Apr 2007 23:07:25 +0200
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Pavel Machek <pavel@....cz>, Kenneth Crudup <kenny@...ix.com>,
	Nick Piggin <npiggin@...e.de>, Mike Galbraith <efault@....de>,
	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Con Kolivas <kernel@...ivas.org>,
	suspend2-devel@...ts.suspend2.net, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: suspend2 merge (was Re: [Suspend2-devel] Re: CFS and suspend2: hang in atomic copy)

On Wednesday, 25 April 2007 22:44, Linus Torvalds wrote:
> 
> On Wed, 25 Apr 2007, Pavel Machek wrote:
> > 
> > Can I get you on IRC somewhere? No, I do not think I'm a moron, and
> > yes, I need to suspend^Wsnapshot the devices before, so I have that in
> > the snapshot. Of course, I'll need to resume^Wrestore the devices
> > before writing snapshot. That's okay, it does not take long.
> 
> You do NOT need to "suspend" the devices, and that's the whole point.
> 
> You may want to save the device info somewhere, BUT THAT IS SOMETHING 
> TOTALLY DIFFERENT!
> 
> This is *exactly* the confusion I'm talking about. The STD and STR 
> codepaths try to use the same function for two TOTALLY DIFFERENT things.
> 
> STR actually wants to "suspend".
> 
> STD actually wants to "atomic snapshot", and it must not allow allocations 
> or anything like that, because the whole snapshot image should be done 
> atomically as one event. But it should *not* suspend, because that device 
> may actually be needed afterwards. 
> 
> So not the same thing at all.
> 
> So here's what "suspend()" wants:
>  - suspend() - preparatory work, can error our, can delay, can park the 
>    disk, etc etc.
>  - suspend_late() - called late, with interrupts disabled, should actually
>    suspend if the early suspend didn't do it already
> 
> And here is what "snapshot()" wants:
>  - prepare_to_snapshot() (for memory allocation)
>  - snapshot() - called late, with interrupts disabled, save state.
> 
> and there is absolutely _zero_ overlap between them. There just isn't 
> anything in common. Yes, both are two-phase (for the simple reason that 
> both want an "atomic" part), but there's really no real overlap.
> 
> Just trying to *make* them be the same operations is just going to 
> introduce flags that then cause them to be totally different *and* 
> confusing and generate bugs. It also means that people do one of them, and 
> "it works" for that case, and the other case is totally broken, but it's 
> not obvious, because doing one means that the system _thinks_ that you did 
> both!
> 
> In the very unlikely case that some driver actually *wants* to use the 
> same function for snapshots and suspending, that driver could just go 
> ahead and _use_ the same function pointer. But now, as things are set up, 
> we force a total confusion on drivers by calling them through the same 
> interface for two totally different things.

I agree, except there are surprisingly many drivers like that.

You're right, we should be doing all of it in a different way, but this means
a lot of changes and we can't do them overnight.

As I wrote in the reply to Pavel, I think we can introduce .freeze(), .thaw()
(and .prethaw() for that matter) callbacks for hibernation and make drivers
use them, but that will be a long series of patches.  Still, I think it's
doable.

Greetings,
Rafael

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