linux-kernel - Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it

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Message-ID: <20061208085634.GA25751@flint.arm.linux.org.uk>
Date:	Fri, 8 Dec 2006 08:56:34 +0000
From:	Russell King <rmk+lkml@....linux.org.uk>
To:	Nick Piggin <nickpiggin@...oo.com.au>
Cc:	Christoph Lameter <clameter@....com>,
	David Howells <dhowells@...hat.com>, torvalds@...l.org,
	akpm@...l.org, linux-arm-kernel@...ts.arm.linux.org.uk,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH] WorkStruct: Implement generic UP cmpxchg() where an arch doesn't support it

On Fri, Dec 08, 2006 at 12:18:52PM +1100, Nick Piggin wrote:
> Russell King wrote:
> >On Thu, Dec 07, 2006 at 08:31:08PM +1100, Nick Piggin wrote:
> 
> >>>Implementing ll/sc based accessor macros allows both ll/sc _and_ cmpxchg
> >>>architectures to produce optimal code.
> >>>
> >>>Implementing an cmpxchg based accessor macro allows cmpxchg architectures
> >>>to produce optimal code and ll/sc non-optimal code.
> >>>
> >>>See my point?
> >>
> >>Wrong. Your ll/sc implementation with cmpxchg is buggy. The cmpxchg
> >>load_locked is not locked at all,
> >
> >
> >Intentional - cmpxchg architectures don't generally have a load locked.
> 
> Exactly, so it is wrong -- you can't implement that behaviour with
> load + cmpxchg.

I disagree.  I _have_ implemented the required behaviour.  I really
don't understand your point saying that it is wrong.

> >>and there can be interleaving writes
> >>between the load and cmpxchg which do not cause the store_conditional
> >>to fail.
> >
> >
> >In which case the cmpxchg fails and we do the atomic operation again,
> >in exactly the same way that we do the operation again if the 'sc'
> >fails in the ll/sc case.
> 
> Not if cmpxchg sees the same value, it won't fail, regardless of how
> many writes have hit that memory address.

Don't see anything wrong with that.  If that was a problem, atomic
implementations using cmpxchg on x86 would be impossible.

I think you're trying to implement ll/sc semantics on CPUs without
ll/sc which is exactly not what I'm trying to do.  I'd argue that's
impossible.

I'm trying to suggest a better implementation for atomic ops rather
than just bowing to this x86-centric "cmpxchg is the best, everyone
must implement it" mentality.

-- 
Russell King
 Linux kernel    2.6 ARM Linux   - http://www.arm.linux.org.uk/
 maintainer of:
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