[Bug 219857] panic in scsi_cd code

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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=219857

Leandro Lupori <leandro.lupori at gmail.com> changed:

           What    |Removed                     |Added
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                 CC|                            |leandro.lupori at gmail.com

--- Comment #28 from Leandro Lupori <leandro.lupori at gmail.com> ---
I was able to reproduce the problem described in comment#14.

Actually, I wasn't trying to reproduce it, I just started seeing it a few weeks
ago, in CURRENT.

I can reproduce it consistently, by using QEMU to run a PowerPC64 VM with a
single core/thread (-smp 1).

It happens only when there is no media in the emulated CD-ROM, a device that
QEMU adds by default, unless -nodefaults is specified in command line.

I've debugged it and this is what I've found:

1- After the CD probe is successful, GEOM will try to open the device, which
will end up calling cdcheckmedia(), that sets CD state to
CD_STATE_MEDIA_PREVENT.
2- Next, scsi_prevent() is executed and succeeds, the CD_FLAG_DISC_LOCKED flag
is set and CD state moves to CD_STATE_MEDIA_SIZE.
3- Next, scsi_read_capacity() is executed and fails, state is set to
CD_STATE_MEDIA_ALLOW, cdmediaprobedone() is called and wakes up cdcheckmedia().
4- Then, when cdstart() is invoked to process CD_STATE_MEDIA_ALLOW, it first
checks if CD_FLAG_DISC_LOCKED is set, and if so skips directly to
CD_STATE_MEDIA_SIZE state. This will repeat the steps of bullet 3, entering an
infinite MEDIA_SIZE command loop.

When there is a least another core/thread, the GEOM thread that performed the
initial cdopen() will get scheduled again, closing the CD device, that will
call cdprevent(PR_ALLOW) that clears the CD_FLAG_DISC_LOCKED flag and breaks
the loop.

So, apparently, the problem is CD_STATE_MEDIA_ALLOW being skipped when
CD_FLAG_DISC_LOCKED is set. If I understand correctly, in this case, the state
should be advanced to CD_STATE_MEDIA size only when the current state is
CD_STATE_MEDIA_PREVENT.

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