linux-kernel - Re: Reference counting of MMC host driver modules

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Date:	Mon, 12 Jan 2009 21:01:56 +0100
From:	Enrik Berkhan <Enrik.Berkhan@...com>
To:	Stefan Richter <stefanr@...6.in-berlin.de>
Cc:	Pierre Ossman <drzeus-mmc@...eus.cx>,
	David Vrabel <david.vrabel@....com>,
	linux-kernel@...r.kernel.org
Subject: Re: Reference counting of MMC host driver modules

Stefan Richter wrote:
> Enrik Berkhan wrote:
> > When removing the mmc_host_driver, everything seems to be fine; the
> > MMC/SD block device has been deactivated by mmc_blk_remove(), that in
> > turn has stopped the queue via mmc_cleanup_queue(). mmc_cleanup_queue()
> > calls blk_cleanup_queue() on the underlying struct request_queue. By
> > this, the reference count of the struct request_queues kboj drops to
> > zero. The MD driver still has the block device open and, actually,
> > things work fine unless the memory of the struct request_queue isn't
> > touched, because it is marked dead. Of course, accessing the MD device
> > returns EIO, but that's fine.
> > 
> > When the mmc_host_driver is reloaded, new struct request_queues will be
> > allocated and with some probability, the old memory will be re-used for
> > them or the old memory locations will be re-used for something else. The
> > key point is that the queues still in use by the MD layer will
> > effectively no longer be marked dead or completely corrupted.
> 
> So in short, the request_queue's reference count goes to zero even
> though something still points to it?

Exactly. AFAICS.

I haven't checked yet if this happens using other block device
infrastructure, too.

Enrik
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