linux-kernel - Re: [PATCH RFC 0/7] x86: convert ticketlocks to C and remove duplicate code

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Message-ID: <4E0257BC.9050509@goop.org>
Date:	Wed, 22 Jun 2011 13:59:40 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...e.hu>,
	the arch/x86 maintainers <x86@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Nick Piggin <npiggin@...nel.dk>,
	Jeremy Fitzhardinge <jeremy.fitzhardinge@...rix.com>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH RFC 0/7] x86: convert ticketlocks to C and remove duplicate
 code

On 06/22/2011 01:19 PM, H. Peter Anvin wrote:
> On 06/22/2011 12:21 PM, Jeremy Fitzhardinge wrote:
>> A friend just pointed out that gcc has a "__sync_fetch_and_add()"
>> intrinsic, which compiles to xadd when used in this context.  What's the
>> general feeling about using these kinds of gcc features?
>>
> In general they are good, IF:
>
> a) they cover all versions of gcc we care about (or we have a fallback),

What is the supported range for these days?

> and
> b) they have the right semantics.

My main concern was making sure that its a strong enough barrier, but
the documentation is pretty explicit about that.

> Using gcc intrinsics can generate better code than we can in inline
> assembly.

It does seem to do a pretty good job; it generates a plain locked add if
you never look at the returned value, for example.

> However, (b) is a killer since gcc doesn't have a way to generate our
> lock prefix annotations, and so it isn't really useful here.

Yeah, I thought about that.  Its a bit unfortunate we're getting into
spinlock code at all on a UP system, but we don't have a mechanism to
stomp locking at a higher level.  (Ignoring all the insane stuff that
happens when the system becomes UP transiently just because all the
other CPUs have been unplugged for suspend, etc; we just shouldn't
bother in that case.)

    J
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