Messages in this thread | | | Date | Sun, 1 Apr 2007 01:14:52 +0100 | From | Alan Cox <> | Subject | Re: broken device locking, sg vs. sg_io on block devices |
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> > > The use of /dev/sg* is still common practice, its invention predates > > > > The /dev/sg interface cannot do the locking. If you use /dev/sg you are > > Again, it doesn't have to. It can pass the locking operations to the > related block device driver.
No it can't. The driver has no idea what the locking rules are for arbitary command blocks send to arbitary devices. /dev/sg is a *raw* interface. You can send anything to anyone, and the locking rules for that are far too complex for a giant morass of kernel code to get added.
The mess begins because you use /dev/sg and put it in a cdrom group instead of using SG_IO on the /dev/sr device. The mess continues because of the user of O_EXCL locking thus forcing re-open/close by HAL instead of fcntl based co-operative locking.
The job of the kernel is not and never has been to anticipate and correct everything stupid someone tries to do in user space.
As I said before the people wanting to arbitrate serial ports got this right in the mid 1970's your situation is not much more complicated, unless you persist in using /dev/sg - which yes does make it hard, but so does writing it in COBOL, or while standing on your head. And the solution to all three cases is the same *DONT DO IT*
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