[ofa-general] [PATCH] [WORKAROUND] CONFIG_PREEMPT_RT and ib_umad_close() issue

[Roland Dreier]

 > When using OFED-1.2.5 based infiniband kernel modules on 2.6.22 based
 > kernels with the Ingo Molnar CONFIG_PREEMPT_RT applied, then commands
 > such as ibnetdiscvoer, smpquery, sminfo, etc. will hang.  The problem
 > is with the downgrade_write() rw semaphore usage in the
 > ib_umad_close() routine.

Can you give a few more details on how PREEMPT_RT changes locking
rules (or just exposes existing bugs maybe?) so that the
downgrade_write() causes the issue?  I would like to fix this cleanly
but I don't really understand what the problem is.

 - R.