linux-kernel - Re: [PATCH v2 1/2] spinlock: New spinlock_refcount.h for lockless update of refcount

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Message-ID: <51CF610B.7090709@hp.com>
Date:	Sat, 29 Jun 2013 18:34:51 -0400
From:	Waiman Long <waiman.long@...com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
CC:	Alexander Viro <viro@...iv.linux.org.uk>,
	Jeff Layton <jlayton@...hat.com>,
	Miklos Szeredi <mszeredi@...e.cz>,
	Ingo Molnar <mingo@...hat.com>,
	linux-fsdevel <linux-fsdevel@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Andi Kleen <andi@...stfloor.org>,
	"Chandramouleeswaran, Aswin" <aswin@...com>,
	"Norton, Scott J" <scott.norton@...com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH v2 1/2] spinlock: New spinlock_refcount.h for lockless
 update of refcount

On 06/29/2013 06:11 PM, Linus Torvalds wrote:
> On Sat, Jun 29, 2013 at 2:34 PM, Waiman Long<waiman.long@...com>  wrote:
>> I think I got it now. For architecture with transactional memory support to
>> use an alternative implementation, we will need to use some kind of dynamic
>> patching at kernel boot up time as not all CPUs in that architecture will
>> have that support. In that case the helper functions have to be real
>> functions and cannot be inlined. That means I need to put the implementation
>> into a spinlock_refcount.c file with the header file contains structure
>> definitions and function prototypes only. Is that what you are looking for?
> Yes. Except even more complex: I want the generic fallbacks in a
> lib/*.c files too.
>
> So we basically have multiple "levels" of specialization:
>
>   (a) the purely lock-based model that doesn't do any optimization at
> all, because we have lockdep enabled etc, so we *want* things to fall
> back to real spinlocks.
>
>   (b) the generic cmpxchg approach for the case when that works
>
>   (c) the capability for an architecture to make up its own very
> specialized version
>
> and while I think in all cases the actual functions are big enough
> that you don't ever want to inline them, at least in the case of (c)
> it is entirely possible that the architecture actually wants a
> particular layout for the spinlock and refcount, so we do want the
> architecture to be able to specify the exact data structure in its own
> <asm/spinlock-refcount.h>  file. In fact, that may well be true of case
> (b) too, as Andi already pointed out that on x86-32, an "u64" is not
> necessarily sufficiently aligned for efficient cmpxchg (it may *work*,
> but cacheline-crossing atomics are very very slow).
>
> Other architectures may have other issues - even with a "generic"
> cmpxchg-based library version, they may well want to specify exactly
> how to take the lock. So while (a) would be 100% generic, (b) might
> need small architecture-specific tweaks, and (c) would be a full
> custom implementation.
>
> See how we do<asm/word-at-a-time.h>  and CONFIG_DCACHE_WORD_ACCESS.
> Notice how there is a "generic"<asm-generic/word-at-a-time.h>  file
> (actually, big-endian only) for reference implementations (used by
> sparc, m68k and parisc, for example), and then you have "full custom"
> implementations for x86, powerpc, alpha and ARM.
>
> See also lib/strnlen_user.c and CONFIG_GENERIC_STRNLEN_USER as an
> example of how architectures may choose to opt in to using generic
> library versions - if those work sufficiently well for that
> architecture. Again, some architecture may decide to write their own
> fully custome strlen_user() function.
>
> Very similar concept.
>
>                   Linus

Thank for the quick response. I now have a much better idea of what I 
need to do. I will send out a new patch for review once the code is ready.

Regards,
Longman
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