linux-kernel - Re: [RFC PATCH] Writer-biased low-latency rwlock v8

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Date:	Thu, 21 Aug 2008 14:26:05 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.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] Writer-biased low-latency rwlock v8

Linus Torvalds wrote:
> 
> Because that is already crap.
> 
> Go look at my code once more. Go look at how it has 128 bits of data, 
> exactly so that it DOES NOT HAVE TO LIMIT THE NUMBER OF READERS.
> 
> And then go look at it again.
> 
> Look at it five times, and until you can understand that it still uses 
> just a 32-bit word for the fast-path and no unnecessarily crap in it, but 
> it actually has 128 bits of data for all the slow paths, don't bother 
> emailing me any new versions.
> 
> Please. You -still- apparently haven't looked at it, at least not enough 
> to understand the _point_ of it. You still go on about trying to fit in 
> three or four different numbers in that one word. Even though the whole 
> point of my rwlock is that you need exactly _one_ count (active writers), 
> and _one_ bit (active reader) and _one_ extra bit ("contention, go to slow 
> path, look at the other bits ONLY IN THE SLOW PATH!")
> 
> That leaves 30 bits for readers. If you still think you need to "limit the 
> number of readers", then you aren't getting it.
> 

First of all, let me say I don't pretend to understand formally how you 
deal with overflow-after-the-fact, as unlikely as it is.

However, it seems to me to be an easy way to avoid it.  Simply by 
changing the read-test mask to $0x80000003, you will kick the code down 
the slow path once the read counter reaches $0x80000004 (2^29+1 
readers), where you can do any necessary fixup -- or BUG() -- at leisure.

This fastpath ends up being identical in size and performance to the one 
you posted, although yours could be reduced by changing the test to a 
testb instruction -- at the almost certainly unacceptable expense of 
taking a partial-register stall on the CPUs that have those.

	-hpa
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