linux-kernel - Re: [ofa-general] [PATCH] [WORKAROUND] CONFIG_PREEMPT_RT and ib_umad

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Date:	Mon, 17 Sep 2007 08:56:01 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	John Blackwood <john.blackwood@...r.com>
Cc:	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	general@...ts.openfabrics.org,
	Sven-Thorsten Dietrich <sdietrich@...ell.com>
Subject: Re: [ofa-general] [PATCH] [WORKAROUND] CONFIG_PREEMPT_RT and ib_umad_close() issue

 > 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.
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