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

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Date:	Sat, 28 Nov 2009 09:30:18 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Nick Piggin <npiggin@...e.de>
cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rfc] "fair" rw spinlocks

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).

That usage case _might_ be turned into RCU or something similar, in which 
case I don't think any major rwlock users remain. However, if that's the 
case, then why should anybody care about fairness any more either?

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.

		Linus
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