linux-kernel - Re: [PATCH] sr: Ensure disk is revalidated when media changes

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Date:	Fri, 8 Apr 2011 17:43:16 +0100
From:	Stefan Hajnoczi <stefanha@...il.com>
To:	Tejun Heo <tj@...nel.org>
Cc:	Amit Shah <amit.shah@...hat.com>, linux-kernel@...r.kernel.org,
	Jens Axboe <axboe@...nel.dk>,
	"James E.J. Bottomley" <James.Bottomley@...e.de>,
	linux-scsi@...r.kernel.org, Markus Armbruster <armbru@...hat.com>
Subject: Re: [PATCH] sr: Ensure disk is revalidated when media changes

On Fri, Apr 8, 2011 at 5:20 PM, Tejun Heo <tj@...nel.org> wrote:
> Hello,
>
> On Fri, Apr 08, 2011 at 12:37:56PM +0100, Stefan Hajnoczi wrote:
>> There is a related issue I have been discussing with Amit:
>> https://lkml.org/lkml/2011/3/23/156
>>
>> On media change the inode size is not updated by the sr driver or the
>> universal cdrom driver.  A userspace process that holds a /dev/sr0
>> file descriptor open across media change causes all processes on the
>> system to see the old medium size when they do lseek(fd, 0, SEEK_END).
>>
>> I think it would make sense to refresh the inode size on media change
>> so that even open file descriptors see the new size and a single
>> process cannot force a stale value for all other userspace processes
>> on the system.
>
> Hmmm... I don't know.  Maybe we can but I'm not sure whether there's a
> good reason for it.  cdrom is locked while opened after all.  Are
> there actual problems?

Yeah, sorry I didn't explain what the use case was.  With QEMU you can
pass through the physical CD-ROM into the virtual machine.

QEMU opens /dev/cdrom with O_NONBLOCK | O_RDONLY.  The guest can test
if the medium is present and QEMU will do ioctl(fd,
CDROM_DRIVE_STATUS, CDSL_CURRENT).  The guest can also lock the tray
and eject, again using the respective ioctls.  Read operations are
serviced by performing a read on the file descriptor in QEMU.  And
finally the medium size is queried by QEMU using lseek(fd, 0,
SEEK_END).

Today QEMU cannot keep /dev/cdrom open across media change because it
will have an outdated inode size returned from lseek(fd, 0, SEEK_END).
 But if the cdrom driver (or sr) refresh the inode size on media
change then there is no need to work around this from userspace.

Stefan
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