On Fri, Apr 8, 2011 at 5:20 PM, Tejun Heo <tj@xxxxxxxxxx> 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 Today QEMU cannot keep /dev/cdrom open across media change because it will have an outdated inode size returned from lseek(fd, 0, SEEK_END).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 -- To unsubscribe from this list: send the line "unsubscribe linux-scsi" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
