linux-kernel - [PATCH 0/3] PM, vfs: use filesystem freezing instead of kthread freezer

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Date:	Fri, 30 Oct 2015 14:47:03 +0100 (CET)
From:	Jiri Kosina <jikos@...nel.org>
To:	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	Dave Chinner <david@...morbit.com>, Jan Kara <jack@...e.cz>,
	Christoph Hellwig <hch@....de>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Al Viro <viro@...IV.linux.org.uk>, Tejun Heo <tj@...nel.org>,
	Pavel Machek <pavel@....cz>
cc:	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: [PATCH 0/3] PM, vfs: use filesystem freezing instead of kthread
 freezer

This series is a followup to my proposal I brought up on Kernel Summit in 
Seoul. Noone seemed to had any principal objections, so let's have wider 
audience look into it.

In a nuthsell: freezing of kernel threads is horrible interface with 
unclear semantics and guarantees, and I am surprised it ever worked 
properly. Plus there are a lot of places that simply use it in a 
completely wrong way (which is not suprising, given the lack of defined 
semantics and requirements).

I've tested this over a series of suspend/resume cycles on several 
machines with at least ext4, btrfs and xfs, and it survived the testing 
without any harm.

Patch 1/3 	implements the actual change, and has a more detailed 
		explanation on "why?" and "how?" questions in the changelog

Patch 2/3	nukes all (hopefully) the calls to freezer from kthreads 
		in Linus' tree (as of 858e904bd71)

Patch 3/3	introduces WARN_ON() if anyone is trying to make use of 
		this again

Open questions / discussion points:

- is the way I am traversing list of superblocks backwards enough to 
  guarantee correct ordering? Especially: does this work as intended for 
  FUSE?

- should freezable workqueues be dealt with the same way? I haven't even 
  started to look into them in a serious way, but it seems like the 
  drivers that are making use of them would actually like to use proper
  PM callbacks instead

-- 
Jiri Kosina
SUSE Labs
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