Date: Fri, 18 Mar 2005 17:02:29 +0100
From: Ingo Molnar <mingo@elte.hu>
To: Bill Huey <bhuey@lnxw.com>
Cc: "Paul E. McKenney" <paulmck@us.ibm.com>, dipankar@in.ibm.com,
       shemminger@osdl.org, akpm@osdl.org, torvalds@osdl.org,
       rusty@au1.ibm.com, tgall@us.ibm.com, jim.houston@comcast.net,
       manfred@colorfullife.com, gh@us.ibm.com, linux-kernel@vger.kernel.org
Subject: Re: Real-Time Preemption and RCU
Message-ID: <20050318160229.GC25485@elte.hu>
References: <20050318002026.GA2693@us.ibm.com> <20050318125641.GA5107@nietzsche.lynx.com> <20050318131729.GB5107@nietzsche.lynx.com>
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* Bill Huey <bhuey@lnxw.com> wrote:

> I'd like to note another problem. Mingo's current implementation of
> rt_mutex (super mutex for all blocking synchronization) is still
> missing reader counts and something like that would have to be
> implemented if you want to do priority inheritance over blocks.

i really have no intention to allow multiple readers for rt-mutexes. We
got away with that so far, and i'd like to keep it so. Imagine 100
threads all blocked in the same critical section (holding the read-lock)
when a highprio writer thread comes around: instant 100x latency to let
all of them roll forward. The only sane solution is to not allow
excessive concurrency. (That limits SMP scalability, but there's no
other choice i can see.)

	Ingo
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