linux-kernel - Re: [PATCH 2.6.19 5/5] fs: freeze

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Message-Id: <200611092321.47728.rjw@sisk.pl>
Date:	Thu, 9 Nov 2006 23:21:46 +0100
From:	"Rafael J. Wysocki" <rjw@...k.pl>
To:	Pavel Machek <pavel@....cz>
Cc:	Alasdair G Kergon <agk@...hat.com>,
	Eric Sandeen <sandeen@...hat.com>,
	Andrew Morton <akpm@...l.org>, linux-kernel@...r.kernel.org,
	dm-devel@...hat.com, Srinivasa DS <srinivasa@...ibm.com>,
	Nigel Cunningham <nigel@...pend2.net>,
	David Chinner <dgc@....com>
Subject: Re: [PATCH 2.6.19 5/5] fs: freeze_bdev with semaphore not mutex

On Thursday, 9 November 2006 22:41, Pavel Machek wrote:
> Hi!
> 
> > > > This is from a work queue, so in fact from a process context, but from
> > > > a process that is running with PF_NOFREEZE.
> > > 
> > > Why not simply &~ PF_NOFREEZE on that particular process? Filesystems
> > > are free to use threads/work queues/whatever, but refrigerator should
> > > mean "no writes to filesystem" for them...
> > 
> > But how we differentiate worker_threads used by filesystems from the
> > other ones?
> 
> I'd expect filesystems to do &~ PF_NOFREEZE by hand.
> 
> > BTW, I think that worker_threads run with PF_NOFREEZE for a reason,
> > but what exactly is it?
> 
> I do not think we had particulary good reasons...

Well, it looks like quite a lot of drivers depend on them, including libata.

I think we can add a flag to __create_workqueue() that will indicate if
this one is to be running with PF_NOFREEZE and a corresponding macro like
create_freezable_workqueue() to be used wherever we want the worker thread
to freeze (in which case it should be calling try_to_freeze() somewhere).
Then, we can teach filesystems to use this macro instead of
create_workqueue().

Having done that we'd be able to drop the freezing of bdevs patch and forget
about the dm-related complexity.

[Still I wonder if the sys_sync() in freeze_processes() is actually safe if
there's a suspended dm device somewhere in the stack, because in the other
case the freezing of bdevs would be no more dangerous than the thing
that we're already doing.]

Greetings,
Rafael


-- 
You never change things by fighting the existing reality.
		R. Buckminster Fuller
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