linux-kernel - Re: [PATCH] libata, freezer: avoid block device removal while system is frozen

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Message-ID: <20131218213936.GA8218@mtj.dyndns.org>
Date:	Wed, 18 Dec 2013 16:39:36 -0500
From:	Tejun Heo <tj@...nel.org>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	Nigel Cunningham <nigel@...elcunningham.com.au>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Jens Axboe <axboe@...nel.dk>, tomaz.solc@...lix.org,
	aaron.lu@...el.com, linux-kernel@...r.kernel.org,
	Oleg Nesterov <oleg@...hat.com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Fengguang Wu <fengguang.wu@...el.com>
Subject: Re: [PATCH] libata, freezer: avoid block device removal while system
 is frozen

Hello, Rafael.

On Wed, Dec 18, 2013 at 10:48:31PM +0100, Rafael J. Wysocki wrote:
> > I see.  In the long term, I think the right thing to do is making the
> > freezer interface more specific so that only the ones which actually
> > need it do so explicitly.  Right now, kernel freezables are
> > conceptually at a very high level - it's a global task attribute and a
> > major knob in workqueue.  I suppose most of that is historical but by
> > perpetuating the model we're encouraging misuse of freezer in large
> > swaths of the kernel.  Even in this specific case, both writeback and
> > jbd workers have no fundamental reason to be freezable and yet
> > they're, eventually developing into totally unnecessary deadlocks.
> 
> You're right, but I'm not sure how we can make the interface for workqueues
> more specific, for example.  I guess we can simply drop create_freezable_workqueue()
> so that whoever wants to create a freezable workqueue has to use the right
> combination of flags.  Can we make it more specific than that?
> 
> BTW, pm_start_workqueue(), which is a legitimate user, doesn't even use that macro. :-)

Yeah, we can just rip out the whole freezer support and let the
caller's pm notification hooks implement it by doing

	workqueue_set_max_active(wq, 0);
	flush_workqueue(wq);

when it needs to "freeze" the workqueue and then reverse it by doing
the following.

	workqueue_set_max_active(wq, WQ_DFL_ACTIVE);

It'll be a bit more code in the specific users but given the
specificity of the usage I think that's the appropriate way to do it.
It'll drop quite a bit of complexity from the core freezer and
workqueue code paths too.

Thanks.

-- 
tejun
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