linux-kernel - Re: [rfc] "fair" rw spinlocks

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Message-ID: <20091130075746.GJ17484@wotan.suse.de>
Date:	Mon, 30 Nov 2009 08:57:46 +0100
From:	Nick Piggin <npiggin@...e.de>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc] "fair" rw spinlocks

On Sun, Nov 29, 2009 at 10:51:22AM -0800, Paul E. McKenney wrote:
> On Sat, Nov 28, 2009 at 09:30:18AM -0800, Linus Torvalds wrote:
> > So as far as I can tell, we have only one real user of rwlocks where 
> > livelocks might be relevant, but that one real user absolutely _requires_ 
> > the unfair behavior.
> 
> But the required unfairness is limited to unconditionally granting
> recursive read requests, right?  If I understand correctly, if a given
> CPU does not already read-hold the lock, then we can safely make that
> CPU wait for a writer that might otherwise be starved.  Again, is there
> another requirement that I am missing?

I think this is the only ordering requirement.

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