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

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Message-ID: <20131217123457.GD29989@htj.dyndns.org>
Date:	Tue, 17 Dec 2013 07:34:57 -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 Tue, Dec 17, 2013 at 03:34:00AM +0100, Rafael J. Wysocki wrote:
> No, it isn't.  [I guess it was originally, but it has not been the case
> for a very long time.]  It is about getting user space interactions (all of

Heh... no wonder people are all so confused about this thing.

> the sysfs/ioctl/mmap/read/write/you-name-it thingies user space can do to
> devices) when we're calling device suspend/resume routines.  The reason is
> that otherwise all of them would have had to do a "oh, are we suspending by
> the way?" check pretty much on every code path that can be triggered by
> user space.

Freezing userland is fine.  I have no problem with that but up until
now the only use case that seems fundamentally valid to me is freezing
IO processing kthread in a driver as a cheap way to implement
suspend/resume.  At this point, given the general level of confusion,
it seems to be costing more than benefiting.

> > Does that mean that it's safe to unfreeze before invoking resume?
> 
> No, it isn't.

So, are you saying it's really about giving device drivers easy way to
implement suspend/resume?  If that's the case, let's please make it
*way* more specific and clear - ie. things like helpers to implement
suspend/resume hooks trivially or whatnot.  Freezable kthreads (and
now workqueues) have been becoming a giant mess for a while now.

Thanks.

-- 
tejun
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