linux-kernel - Re: Writeback threads and freezable

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Message-ID: <20131216125604.GD32509@htj.dyndns.org>
Date:	Mon, 16 Dec 2013 07:56:04 -0500
From:	Tejun Heo <tj@...nel.org>
To:	Dave Chinner <david@...morbit.com>
Cc:	"Rafael J. Wysocki" <rjw@...k.pl>, 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: Writeback threads and freezable

On Mon, Dec 16, 2013 at 07:51:24AM -0500, Tejun Heo wrote:
> On Mon, Dec 16, 2013 at 02:56:52PM +1100, Dave Chinner wrote:
> > > What are you suggesting?  Implementing separate warm and hot unplug
> > > paths?  That makes no sense whatsoever.  Device hot unplug is just a
> > > sub operation of general device unplug which should be able to succeed
> > > whether the underlying device is failing IOs or not.
> > 
> > I don't care. Trying to issue IO from an an IO error handling path
> > where the device has just been removed is fundamentally broken.
> 
> What?  Have you even read the original message?  IO error handling
> path isn't issuing the IO here.  The hot unplug operation is
> completely asynchronous to the IO path.  What's dead locking is not
> the filesystem and IO path but device driver layer and hot unplug
> path.  IOs are not stalled.

In fact, this deadlock can be reproduced without hotunplug at all.  If
you initiate warm unplug and warm unplugging races with suspend/resume
cycle, it'll behave exactly the same - the IOs from flushing in that
scenario would succeed but IOs failing or not has *NOTHING* to do with
this deadlock.  It'd still happen.  It's freezer behaving as a giant
lock and other locks of course getting dragged into giant dependency
loop.  Can you please at least *try* to understand what's going on
before throwing strong assertions?

-- 
tejun
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