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

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Date:	Mon, 17 Sep 2007 10:07:56 -0700
From:	Daniel Walker <dwalker@...sta.com>
To:	Roland Dreier <rdreier@...co.com>
Cc:	John Blackwood <john.blackwood@...r.com>,
	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

On Mon, 2007-09-17 at 08:56 -0700, Roland Dreier wrote:
> > 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.

the read/write semaphore functionality is basically reduced to just a
binary semaphore , i.e. one reader, or one writer . I think the BUG();
in downgrade_write() is likely part of a removal plan for
downgrade_write() (that's just a guess tho)

Daniel

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