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Date: Tue, 5 Jan 2010 08:14:51 -0800 (PST)
From: Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: [RFC][PATCH 6/8] mm: handle_speculative_fault()
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On Tue, 5 Jan 2010, Linus Torvalds wrote:
> 
> Lookie here:
> 
>  - arch/x86/Kconfig.cpu:
> 
> 	config X86_XADD
> 		def_bool y
> 		depends on X86_32 && !M386
> 
>  - arch/x86/Kconfig:
> 
> 	config RWSEM_GENERIC_SPINLOCK
> 	        def_bool !X86_XADD
> 
> 	config RWSEM_XCHGADD_ALGORITHM
> 	        def_bool X86_XADD   
> 
> it looks like X86_XADD only gets enabled on 32-bit builds. Which means 
> that x86-64 in turn seems to end up always using the slower "generic 
> spinlock" version.

Sadly, it's not as easy as just changing the X86_XADD "depends on" to say 
"X86_64 || !M386" instead. That just results in

	kernel/built-in.o: In function `up_read':
	(.text+0x2d8e5): undefined reference to `call_rwsem_wake'

etc, because the x86-64 code has obviously never seen the optimized 
call-paths, and they need the asm wrappers for full semantics.

Oh well. Somebody who is bored might look at trying to make the wrapper 
code in arch/x86/lib/semaphore_32.S work on x86-64 too. It should make the 
successful rwsem cases much faster.

		Linus

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