Reproducible SMP kernel deadlock in SCSI generic driver (sg)

[Jurgen Botz]

The sg module reproducibly deadlocks the kernel for me after some time
of heavy I/O on an SMP system.  This appears to be true in /all/ kernel
versions... I can reproduce it very reliably now in 2.4.19-pre8 and
2.5.13, and I've had problems with CD ripping on my SMP workstation at
least throughout the 2.4 series (I just never fully investigated before).
The bug is almost certainly in sg.c; here is what I've narrowed down...

- Deadlock when ripping CDs using generic device under SMP after some
  amount of heavy I/O; higher transfer rate seems to make it happen
  sooner.
- Happens with SCSI CD drives as well as IDE drives using ide-scsi or
  USB drives using usb-storage.
- Deadlock happens sooner with usb-storage than with real SCSI device,
  but will eventually happen in either case.  In worst case I've seen
  the deadlock after ~100MB transferred, in the best case after ~3-4GB
  (i.e. ripped about 5-6 CDs).
- No deadlock when using sr device on SMP kernel
- No deadlock with sg or sr on UP kernel

Searching lkml didn't turn up any recent reports of anything like this,
but I suspect that's because not too many people are ripping CDs on
SMP systems these days... however, if anyone out there /does/ and
doesn't see lockups, please let me know.

:j

-- 
Jürgen Botz                       | While differing widely in the various
[e-mail]                   | little bits we know, in our infinite
                                  | ignorance we are all equal. -Karl Popper