linux-kernel - Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless update of refcount

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Date:	Tue, 3 Sep 2013 14:05:38 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Al Viro <viro@...iv.linux.org.uk>,
	Sedat Dilek <sedat.dilek@...il.com>,
	Waiman Long <waiman.long@...com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Jeff Layton <jlayton@...hat.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Arnaldo Carvalho de Melo <acme@...radead.org>
Subject: Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless
 update of refcount

On Tue, Sep 3, 2013 at 12:19 PM, Ingo Molnar <mingo@...nel.org> wrote:
>
> * Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>
>>
>>  - the lglock data structure isn't a percpu data structure, it's this
>> stupid global data structure that has a percpu pointer in it.  So that
>> first "mov (%rdi),%rdx" is purely to load what is effectively a constant
>> address (per lglock).
>>
>>    And that's not because it wants to be, but because we associate
>> global lockdep data with it. Ugh. If it wasn't for that, we could just
>> make them percpu.
>
> I don't think that's fundamental - the per CPU lock was percpu before:
[...]
> but AFAICS got converted to a pointer via this commit:
>
>  commit eea62f831b8030b0eeea8314eed73b6132d1de26
>  Author: Andi Kleen <ak@...ux.intel.com>
>  Date:   Tue May 8 13:32:24 2012 +0930
>
>     brlocks/lglocks: turn into functions

So instead of reverting that entirely, how about making "struct
lglock" always per entirely per-cpu, and replacing the percpu pointer
with the lock itself.

Then, we say "the lockdep map is always on CPU#0".

TOTALLY UNTESTED PATCH ATTACHED. It compiles in at least a couple of
configurations, and I checked that this removes _one_ of the
indirections (the other one is because we don't have a native per-cpu
spinlock helper function, so we need to do that percpu base addition),
but I haven't dared try to actually try to boot it.

Comments?

I'll try booting it and seeing if it actually works (and if it makes
any difference), but it seems to be a reasonable approach. I think it
actually cleans things up a bit, but maybe that's just because I
touched the code now.

            Linus

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