linux-kernel - Re: x86-32: clean up rwsem inline asm statements

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Message-ID: <4B4D1865.5000107@zytor.com>
Date:	Tue, 12 Jan 2010 16:48:37 -0800
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: x86-32: clean up rwsem inline asm statements

On 01/12/2010 04:21 PM, Linus Torvalds wrote:
> 
> So instead of using "(%%eax)" to specify the memory address that is the 
> semaphore, we use "(%1)" or similar. And instead of forcing the operation 
> to always be 32-bit, we use "%z0", taking the size from the actual 
> semaphore data structure itself.
> 
> This doesn't actually matter on x86-32, but if we want to use the same 
> inline asm for x86-64, we'll need to have the compiler generate the proper 
> 64-bit names for the registers (%rax instead of %eax), and if we want to 
> use a 64-bit counter too (in order to avoid the 15-bit limit on the 
> write counter that limits concurrent users to 32767 threads), we'll need 
> to be able to generate instructions with "q" accesses rather than "l".
> 

As far as I can tell, very few of these assembly statements actually
need a size at all -- the very first inc statement is purely to memory,
and as such it needs a size marker, but almost any operation which is
register-register or register-memory will simply take its size from the
register operand.  For those, it seems cleaner to simply drop the size
suffix, and in fact this is the style we have been pushing people
towards (use the suffix where mandatory or where the size is fixed
anyway, to help catch bugs; use no suffix where the size can vary and is
implied by the operands.)

So, proposed alternate version of your patch attached.

(Also changed cmpxchgl -> cmpxchg.)

	-hpa

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