Re: [PATCH] Remove process freezer from suspend to RAM pathway

[Nigel Cunningham <nigel@nigel.suspend2.net>]

Hi.

On Friday 06 July 2007 08:46:54 Benjamin Herrenschmidt wrote:
> On Thu, 2007-07-05 at 11:30 +0200, Pavel Machek wrote:
> > 
> > ...but the moment you start blocking tasks that done driver request,
> > you _do_ have mini-freezer of your own, with pretty much the same
> > problems.
> 
> No, not at all the same problems. Those tasks will block, but that will
> be harmless because we won't have some "freezer" things waiting for all
> tasks to reach a "stable" point (calling try_to_freeze()). We just let
> them block wherever we want, as long as it doesn't prevent a -driver-
> from suspending, which should be allright, we have no problem.

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?

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.

> > In another message I shown that removing freezer will not help with
> > FUSE in general case.
> 
> I disagree.

Why?
 
> > It probably does not help with firmware, too; as soon as udev attempts
> > to do something with your wireless card, it is blocked, and if the
> > wireless card needs the firmware from udev, you are deadlocked.
> 
> Firmware load has been a problem since day 1, I've talked about it
> multiple times, it's broken with or without the freezer, and so far, the
> reaction of pretty much everybody has been to dig their head deeper in
> the mud and ignore the problem.
> 
> There are other issues (again, with or without freezer) that should be
> dealt with. For example, drivers that haven't yet got their suspend()
> callback or already have got their resume() may rely on services of the
> kernel that are still blocked, that's where things may go hairy.
> request_firmware() within resume() is a typical example of that.
> 
> There are a few things we should do in that area. For example, once we
> start to call driver suspend's, we should probably set a system wide
> flag that will do things such as:
> 
>  - block usermode helpers (either make call_usermodehelper return
> something like -EBUSY or have it queue up the calls and issue them later
> when thing are resuming, we need to look closely at what semantic we
> want here).
> 
>  - 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?)
 
>  - 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...
> 
> 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?

Regards,

Nigel

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