linux-kernel - Re: Writeback threads and freezable

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Message-ID: <1639602.dY4DbMiF4F@vostro.rjw.lan>
Date:	Wed, 18 Dec 2013 23:14:24 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Tejun Heo <tj@...nel.org>
Cc:	Dave Chinner <david@...morbit.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: Writeback threads and freezable

On Wednesday, December 18, 2013 06:43:43 AM Tejun Heo wrote:

[...]

> If filesystems need an indication that the underlying device is no
> longer functional, please go ahead and add it, but please keep in mind
> all these are completely asynchronous.  Nothing guarantees you that
> such events would happen in any specific order.  IOW, you can be at
> *ANY* point in your warm unplug path and the device is hot unplugged,
> which essentially forces all the code paths to be ready for the worst,
> and that's exactly why there isn't much effort in trying to separate
> out warm and hot unplug paths.

Yes.  Devices can go away at any point without notice.  Even PCI devices
that have never been assumed to be hot-removable.  Any piece of code in the
kernel needs to be prepared to deal with such situations.

Thanks,
Rafael
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