linux-kernel - How to cleanly shut down a block device

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Message-ID: <455969F2.80401@drzeus.cx>
Date:	Tue, 14 Nov 2006 08:02:10 +0100
From:	Pierre Ossman <drzeus-list@...eus.cx>
To:	Jens Axboe <axboe@...nel.dk>, LKML <linux-kernel@...r.kernel.org>
Subject: How to cleanly shut down a block device

Hi Jens,

I've been trying to sort out some bugs in the MMC layer's block driver,
but my knowledge about the block layer is severely lacking. So I was
hoping you could educate me a bit. :)

Upon creation, the following happens:

alloc_disk()
spin_lock_init()
blk_init_queue()
blk_queue_*() (Set up limits)
disk->* = * (assign members)
blk_queue_hardsect_size()
set_capacity()
add_disk()

And on a clean removal, where there are no users of a card when it is
removed:

del_gendisk()
put_disk()
blk_cleanup_queue()

So far everything seems nice and peachy. The question is what to do when
a card is removed when the device is open.

In that case, del_gendisk() will be called, which seems to be documented
as blocking any new requests to be added to the queue. But there will be
a lot of outstanding requests in the queue.

Is it up to each block device to iterate and fail these or can I tell
the kernel "I'm broken, go away!"?

When the queue eventually drains (without too many oopses) and the user
calls close(), then put_disk() and blk_cleanup_queue() will be called.

Rgds
-- 
     -- Pierre Ossman

  Linux kernel, MMC maintainer        http://www.kernel.org
  PulseAudio, core developer          http://pulseaudio.org
  rdesktop, core developer          http://www.rdesktop.org
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