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

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Message-ID: <20151030174417.GA16781@amd>
Date:	Fri, 30 Oct 2015 18:44:17 +0100
From:	Pavel Machek <pavel@....cz>
To:	Alan Stern <stern@...land.harvard.edu>
Cc:	Jiri Kosina <jikos@...nel.org>,
	"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>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	linux-pm@...r.kernel.org
Subject: Re: [PATCH 0/3] PM, vfs: use filesystem freezing instead of kthread
 freezer

On Fri 2015-10-30 11:29:08, Alan Stern wrote:
> On Fri, 30 Oct 2015, Jiri Kosina wrote:
> 
> > 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
> 
> This patch talks about freezing in relation to hibernation.  What about 
> other forms of suspend?
> 
> Also, it replaces kthread freezing with filesystem freezing.  What 
> about kthreads performing I/O that doesn't go through a filesystem?  
> You write:
> 
> > the only facility that is needed during suspend: "no persistent fs
> > changes are allowed from now on"
> 
> I would say instead "no I/O is allowed from now on".  Maybe that's an 
> overstatement, but I think it comes closer to the truth.

Exactly. And I'm pretty sure hardware drivers do use kernel threads,
and do I/O from them.

LEDs are just one example

Best regards,
								Pavel

-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html
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