linux-kernel - Re: Getting rid of freezer for suspend [was Re: [fuse-devel] [PATCH] fuse: make fuse daemon frozen along with kernel threads]

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Message-ID: <2407765.5u9DGJxmKr@vostro.rjw.lan>
Date:	Mon, 11 Feb 2013 20:28:46 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Miklos Szeredi <miklos@...redi.hu>
Cc:	Pavel Machek <pavel@....cz>,
	Goswin von Brederlow <goswin-v-b@....de>,
	Li Fei <fei.li@...el.com>, len.brown@...el.com,
	mingo@...hat.com, peterz@...radead.org, biao.wang@...el.com,
	linux-pm@...r.kernel.org, fuse-devel@...ts.sourceforge.net,
	linux-kernel@...r.kernel.org, chuansheng.liu@...el.com
Subject: Re: Getting rid of freezer for suspend [was Re: [fuse-devel] [PATCH] fuse: make fuse daemon frozen along with kernel threads]

On Monday, February 11, 2013 02:59:56 PM Miklos Szeredi wrote:
> On Mon, Feb 11, 2013 at 1:08 PM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> > On Monday, February 11, 2013 11:11:40 AM Miklos Szeredi wrote:
> >> On Mon, Feb 11, 2013 at 12:31 AM, Rafael J. Wysocki <rjw@...k.pl> wrote:
> >> > On Sunday, February 10, 2013 07:55:05 PM Pavel Machek wrote:
> >>
> >> >> Well, from freezer you need:
> >> >>
> >> >> 1) user process frozen.
> >> >>
> >> >> 2) essential locks _not_ held so that block devices are still functional.
> >> >>
> >> >> > > > mmap... what is problem with mmap? For suspend, memory is powered, so
> >> >> > > > you can permit people changing it.
> >> >> > >
> >> >> > > Suppose mmap is used to make the registers of some device available to user
> >> >> > > space.  Yes, that can happen.
> >> >>
> >> >> "Don't do it, then". Yes, can happen, but hopefully is not too common
> >> >> these days. [And... freezer doing 1) but not 2) would be enough to
> >> >> handle that. Freezer doing 1) but not 2) would also be simpler...]
> >> >
> >> > Again, I'm not sure what you mean.
> >> >
> >> > Are you trying to say that it would be OK to freeze user space tasks in
> >> > the D state?
> >>
> >> I think that's what Pavel is saying.   Processes in D state sleeping
> >> on non-device mutexes _are_ actually OK to freeze.  And that would
> >> nicely solve the fuse freeze problem.
> >
> > That's potentially deeadlock-prone, because a task waiting for mutex X may
> > very well be holding mutex Y, so if there's another task waiting for mutex Y,
> > it needs to be frozen at the same time.
> >
> >> The only little detail is how do we implement that...
> >
> > This means the only way I can see would be to hack the mutex code so that the
> > try_to_freeze() was called for user space tasks after the
> > sched_preempt_enable_no_resched() before schedule().
> >
> > That shouldn't be a big deal performance-wise, because we are in the slow
> > path anyway then.  I'm not sure if Peter Z will like it, though.
> >
> > Moreover, a task waiting for a mutex may be holding a semaphore or be
> > participating in some other mutual-exclusion mechanism, so we'd need to
> > address them all.  Plus, as noted by Pavel, freezing those things would make
> > it difficult to save hibernation images to us.
> 
> Well, as a first step I could cook up a patch that adds a flag to the
> mutex indicating that it's freezable.  Fuse would mark its mutexes
> (and the mutexes that VFS uses on its behalf) as freezable.  That way
> we don't interfere with hibernation, except if hibernation uses fuse
> but that's a very special case.

Yes, that may be worth a shot. :-)

Thanks,
Rafael


-- 
I speak only for myself.
Rafael J. Wysocki, Intel Open Source Technology Center.
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