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

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Message-ID: <20091130100041.GA29610@infradead.org>
Date:	Mon, 30 Nov 2009 05:00:41 -0500
From:	Christoph Hellwig <hch@...radead.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Nick Piggin <npiggin@...e.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc] "fair" rw spinlocks

On Sat, Nov 28, 2009 at 09:30:18AM -0800, Linus Torvalds wrote:
> 
> 
> On Mon, 23 Nov 2009, Nick Piggin wrote:
> > 
> > Last time this issue came up that I could see, I don't think
> > there were objections to making rwlocks fair, the main
> > difficulty seemed to be that we allow reentrant read locks
> > (so a write lock waiting must not block arbitrary read lockers).
> 
> We have at least one major rwlock user - tasklist_lock or whatever. And 
> that one definitely depends on being able to do 'rwlock()' in an 
> interrupt, without other rwlock'ers having to disable irq's (even if there 
> might be a new writer coming in on another cpu).

How long will this use be around?  I've seen some slow progress toward
replacing most read side uses of the task list lock with RCU.  While we
still have lots of read side users now I wonder when they'll go away.

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