linux-kernel - Re: [PATCH 01/10] x86 rwsem: minor cleanups

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Date:	Thu, 20 May 2010 14:37:11 -0700
From:	Michel Lespinasse <walken@...gle.com>
To:	David Howells <dhowells@...hat.com>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Mike Waychison <mikew@...gle.com>,
	Suleiman Souhlal <suleiman@...gle.com>,
	Ying Han <yinghan@...gle.com>
Subject: Re: [PATCH 01/10] x86 rwsem: minor cleanups

On Wed, May 19, 2010 at 4:47 AM, David Howells <dhowells@...hat.com> wrote:
> Given that you describe this first, this would suggest that the subject of the
> patch should be this.  I'm not sure I'd count this as a minor cleanup.  I think
> I'd split it into its own patch.

OK, split it in two.

> Mostly okay, except where you said "expects old value in %edx" - that's only
> true on i386, not x86_64.  On the latter it would be %rdi.  However, I can live
> with that: it's true enough.

It's actually still %edx in x86_64 - we're calling into
arch/x86/lib/rwsem_64.S which has its own unusual conventions.

>> -     rwsem_count_t tmp;
>> +     rwsem_count_t tmp = -RWSEM_ACTIVE_WRITE_BIAS;
>> ...
>>                    : "+m" (sem->count), "=d" (tmp)
>> -                  : "a" (sem), "1" (-RWSEM_ACTIVE_WRITE_BIAS)
>> +                  : "a" (sem), "1" (tmp)
>
> If you're going to put the initialisation of EDX/RDI on tmp (which isn't really
> necessary), rather than directly on the asm statement, you could change the
> '"=d" (tmp)' output constraint to be '"+d" (tmp)' and drop the '"1" (tmp)'
> constraint entirely.

I agree it'd be nicer, but I wondered if all gcc versions would handle
the constraints change fine and then I chickened out. Instead I moved
the initialization on the constraints list as was already done in
__up_write(). All I was really shooting for here is consistency
accross __down_write_nested, __up_read and __up_write functions.

> However, apart from that, feel free to add my Acked-by to this patch or its
> split resultant patches.

Thanks. I'll send a V4 series soon integrating your feedback & mark
the two splitted patches resulting from this one as Acked-by.

-- 
Michel "Walken" Lespinasse
A program is never fully debugged until the last user dies.
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