linux-kernel - Re: [patch 01/28] Fall back on interrupt disable in cmpxchg8b on 80386 and 80486

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Date:	Tue, 28 Aug 2007 11:40:26 +1000
From:	Nick Piggin <nickpiggin@...oo.com.au>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
CC:	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
	clameter@....com, mingo@...hat.com
Subject: Re: [patch 01/28] Fall back on interrupt disable in cmpxchg8b on
 80386 and 80486

Mathieu Desnoyers wrote:

> Q:
> What's the reason to have cmpxchg64_local on 32 bit architectures?
> Without that need all this would just be a few simple defines.
> 
> A:
> cmpxchg64_local on 32 bits architectures takes unsigned long long
> parameters, but cmpxchg_local only takes longs. Since we have cmpxchg8b
> to execute a 8 byte cmpxchg atomically on pentium and +, it makes sense
> to provide a flavor of cmpxchg and cmpxchg_local using this instruction.
> 
> Also, for 32 bits architectures lacking the 64 bits atomic cmpxchg, it
> makes sense _not_ to define cmpxchg64 while cmpxchg could still be
> available.
> 
> Moreover, the fallback for cmpxchg8b on i386 for 386 and 486 is a
> different case than cmpxchg (which is only required for 386). Using
> different code makes this easier.
> 
> However, cmpxchg64_local will be emulated by disabling interrupts on all
> architectures where it is not supported atomically.
> 
> Therefore, we *could* turn cmpxchg64_local into a cmpxchg_local, but it
> would make the 386/486 fallbacks ugly, make its design different from
> cmpxchg/cmpxchg64 (which really depends on atomic operations and cannot
> be emulated) and require the __cmpxchg_local to be expressed as a macro
> rather than an inline function so the parameters would not be fixed to
> unsigned long long in every case.
> 
> So I think cmpxchg64_local makes sense there, but I am open to
> suggestions.

Every new thing like this (especially 64 bit operation on 32 bit
architectures) adds a tiny bit more burden for maintainers. Are
there any callers? If not, don't add it. It's simple to add if we
do get a good reason.

-- 
SUSE Labs, Novell Inc.
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