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

From: Pavel Machek <pavel@ucw.cz>
To: Alan Stern <stern@rowland.harvard.edu>
Cc: Jiri Kosina <jikos@kernel.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>,
	Dave Chinner <david@fromorbit.com>, Jan Kara <jack@suse.cz>,
	Christoph Hellwig <hch@lst.de>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Al Viro <viro@ZenIV.linux.org.uk>, Tejun Heo <tj@kernel.org>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-pm@vger.kernel.org
Subject: Re: [PATCH 0/3] PM, vfs: use filesystem freezing instead of kthread freezer
Date: Fri, 30 Oct 2015 18:44:17 +0100	[thread overview]
Message-ID: <20151030174417.GA16781@amd> (raw)
In-Reply-To: <Pine.LNX.4.44L0.1510301121340.1659-100000@iolanthe.rowland.org>

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

-- 
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