linux-kernel - Re: 3.11-rc regression bisected: s2disk does not work (was Re: [PATCH v3 13/16] futex: use freezable blocking call)

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Message-ID: <CAMbhsRReF9xB597i9CcCj7D1P5kvB4cc0JmDQYeboqi11Kp99A@mail.gmail.com>
Date:	Mon, 22 Jul 2013 17:42:49 -0700
From:	Colin Cross <ccross@...roid.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Michael Leun <lkml20130126@...ton.leun.net>,
	lkml <linux-kernel@...r.kernel.org>, Pavel Machek <pavel@....cz>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mandeep Singh Baines <msb@...omium.org>,
	Oleg Nesterov <oleg@...hat.com>,
	linux-nfs <linux-nfs@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	netdev <netdev@...r.kernel.org>, Tejun Heo <tj@...nel.org>,
	Darren Hart <dvhart@...ux.intel.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Randy Dunlap <rdunlap@...radead.org>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: 3.11-rc regression bisected: s2disk does not work (was Re: [PATCH
 v3 13/16] futex: use freezable blocking call)

On Mon, Jul 22, 2013 at 5:32 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Mon, Jul 22, 2013 at 4:55 PM, Colin Cross <ccross@...roid.com> wrote:
>>
>> I think the right solution is to add a flag to the freezing task that
>> marks it unfreezable.  I  think PF_NOFREEZE would work, although it is
>> normally used on kernel threads, can you see if the attached patch
>> helps?
>
> Hmm. That does seem to be the right thing to do, but I wonder about
> the *other* callers of freeze_processes() IOW, kexec and friends.
>
> So maybe we should do this in {freeze|thaw}_processes() itself, and
> just make the rule be that the caller of freeze_processes() itself is
> obviously not frozen, and has to be the same one that then thaws
> things?
>
> Colin? Rafael? Comments?
>
>                 Linus

I was worried about clearing the flag in thaw_processes().  If a
kernel thread with PF_NOFREEZE set ever called thaw_processes(), which
autosleep might do, it would clear the flag.  Or if a different thread
called freeze_processes() and thaw_processes().  All the other callers
besides the SNAPSHOT_FREEZE ioctl stay in the kernel between
freeze_processes() and thaw_processes(), which makes the fanout of
places that could call try_to_freeze() much more controllable.

Using a new flag that operates like PF_NOFREEZE but doesn't conflict
with it, or a nofreeze_depth counter, would also work.
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