linux-kernel - Re: [PATCH] PREEMPT_RT

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Date:	Tue, 6 Sep 2011 11:25:41 +0200
From:	John Kacur <jkacur@...hat.com>
To:	Thomas Gleixner <tglx@...utronix.de>
Cc:	rt-users <linux-rt-users@...r.kernel.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] PREEMPT_RT_FULL Build error fix

On Thu, Sep 1, 2011 at 9:14 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Fri, 26 Aug 2011, John Kacur wrote:
>
>> Problem:
>> make O=/bld/3.0.3-rt11/ kernel/fork.o
>> /home/jkacur/jk-2.6/kernel/fork.c:91: error: section of ‘name’ conflicts with previous declaration
>>
>> __cacheline_aligned is already part of the definition of DEFINE_RWLOCK
>> for -rt
>>
>> However it is not always used for the tasklist_lock in non-rt, so it can't
>> simply be added to the definition of DEFINE_RWLOCK in non-rt, so I modified
>> the definition in fork.c
>
> -ENOPARSE
>
> The real question is whether RWLOCKS should be cacheline aligned in
> general or the RT addon is just overkill. Slapping an ifdef around
> does not answer that.
>

Right, this was just a "put out a build-break fire" for now patch and
not a good long term patch. I noticed that of all the mainline
DEFINE_RWLOCKs that only the tasklist_lock is cache aligned. I'm
wondering if there is a reason for that, or just more of a historical
accident.

Regarding cache aligning the rwlocks in -rt, I also can't find the
historical reason for it, do you remember if it was for latency
performance, or did we have a atomicity issue on some architectures
when we didn't cache align it?

Thanks
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