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

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Date:	Mon, 17 Sep 2007 14:40:08 -0700
From:	Roland Dreier <rdreier@...co.com>
To:	john.blackwood@...r.com
Cc:	linux-rt-users@...r.kernel.org, linux-kernel@...r.kernel.org,
	Sven-Thorsten Dietrich <sdietrich@...ell.com>,
	general@...ts.openfabrics.org
Subject: Re: [ofa-general] [PATCH] [WORKAROUND] CONFIG_PREEMPT_RT and ib_umad_close() issue

Thanks for the explanation...

 > But basically, with CONFIG_PREEMPT_RT enabled, the lock points, such as
 > aqcuiring a spinlock, potentially become places where the current task
 > may be context switched out / preempted.
 > 
 > Therefore, when a call is made to lock a spinlock for example, the
 > caller should not currently have irqs disabled, or preemption disabled,
 > since a context switch may occur.

this doesn't seem relevant here...

 > void fastcall rt_downgrade_write(struct rw_semaphore *rwsem)
 > {
 >         BUG();
 > }

this seems to be the problem... the -rt patch turns downgrade_write()
into a BUG().

I need to look at the locking in user_mad.c again, but I think it may
be possible to replace both places that do downgrade_write() with
up_write() followed by down_read().

 - R.
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