Bug 156576

Description of problem:

dm-mpath paniced at dm_pg_init_complete+0x10 while testing multipath
reaction to CLARiion CX300 non-destructive ucode upgrade called an NDU.

Using version 0.4.3-pre5 multipath tools and version 2.6.11-rc3-udm2
linux kernel.  I see no code in place that would prevent this problem
from occurring on Red Hat AS 4 Update 1.

The panic is occurring due to corrupted memory in the path structure
for a multipath pg_init i/o completion.  I suspect that the memory for
the path structure (and its encompassing path group and multipath structure)
has been freed by the multipath destructor, subsequently re-allocated
for other use, and written upon.

I suspect that the problem is caused by having a pg_init request oustanding
while the pending count on a multipath mapped device is zero when the
multipath mapped device is suspended.

Since pg_init requests are not accounted for in the pending
count of the multipath mapped device structure, it is possible
to have outstanding pg_init requests awaiting i/o completion
when the pending count is zero.  If the multipath table
is destroyed via dm_table_destroy() before the pg_init i/o
completion arrives, dm_pg_init_complete() can reference
corrupted memory.  This can happen either from the swap-in
of a new dm table or the closing of the dm mapped device.
My panic involves the former use case.

I suspect that the prerequisite state of having a pg_init request
outstanding while having a 0 pending count for a multiapth during
a multipath device suspension is achieved in one of two possible ways.
First, when a multipath, which has no pending requests and a call from
user space to switch_pg_num() sends a pg_init request just prior to having
the multipath device suspended.  Second, when a multipath mapped device
which has one or more pending requests and a pg_init request outstanding
is suspended.

Reasonable fixes for both cases (which I tried) are (1) to only send a pg_init
request if there are one or more pending requests queued for the multipath and 
(2) to not dispatch pending requests if there is a pg_init request outstanding.
With these changes in place, I no longer incurred this problem.

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