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

From: Tejun Heo
Date: Fri Apr 08 2011 - 12:52:17 EST


On Fri, Apr 08, 2011 at 05:43:16PM +0100, Stefan Hajnoczi wrote:
> >> 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,
> 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 will have an outdated inode size returned from lseek(fd, 0, SEEK_END).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.

Hmmm... ISTR there was some discussion about changing inode size on
the fly quite a while ago. I didn't follow the discussion but it
seemed to have rather nasty/delicate implications.

Jens, any ideas?


To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at
Please read the FAQ at