linux-kernel - Re: [RFC PATCH] Fair low-latency rwlock v5

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Message-ID: <alpine.LFD.1.10.0808190946560.3324@nehalem.linux-foundation.org>
Date:	Tue, 19 Aug 2008 09:48:32 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
cc:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Jeremy Fitzhardinge <jeremy@...p.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>, Joe Perches <joe@...ches.com>,
	linux-kernel@...r.kernel.org, Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [RFC PATCH] Fair low-latency rwlock v5



On Tue, 19 Aug 2008, Mathieu Desnoyers wrote:
> 
> It strikes me that Intel has a nice (probably slow?) cmpxchg16b
> instruction on x86_64. Therefore, we could atomically update 128 bits,
> which gives the following table :

Stop this crapola.

Take a look at the rwsem thing I pointed you to. After you understand 
that, come back.

The WHOLE POINT of that thing was to use only 32-bit atomics on the hot 
path. Don't even start think9ing about cmpxchg16b. If you cannot do your 
atomics in 32-bit, they're broken.

Please. I realize that the rwsem implementation I did is subtle. But 
really. Spend the time to understand it.

			Linus
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