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

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Date:	Fri, 30 Aug 2013 09:55:25 +0200
From:	Sedat Dilek <sedat.dilek@...il.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Waiman Long <waiman.long@...com>, Ingo Molnar <mingo@...nel.org>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Alexander Viro <viro@...iv.linux.org.uk>,
	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>
Subject: Re: [PATCH v7 1/4] spinlock: A new lockref structure for lockless
 update of refcount

On Fri, Aug 30, 2013 at 5:54 AM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Aug 29, 2013 at 8:12 PM, Waiman Long <waiman.long@...com> wrote:
>> On 08/29/2013 07:42 PM, Linus Torvalds wrote:
>>>
>>> Waiman? Mind looking at this and testing? Linus
>>
>> Sure, I will try out the patch tomorrow morning and see how it works out for
>> my test case.
>
> Ok, thanks, please use this slightly updated patch attached here.
>
> It improves on the previous version in actually handling the
> "unlazy_walk()" case with native lockref handling, which means that
> one other not entirely odd case (symlink traversal) avoids the d_lock
> contention.
>
> It also refactored the __d_rcu_to_refcount() to be more readable, and
> adds a big comment about what the heck is going on. The old code was
> clever, but I suspect not very many people could possibly understand
> what it actually did. Plus it used nested spinlocks because it wanted
> to avoid checking the sequence count twice. Which is stupid, since
> nesting locks is how you get really bad contention, and the sequence
> count check is really cheap anyway. Plus the nesting *really* didn't
> work with the whole lockref model.
>
> With this, my stupid thread-lookup thing doesn't show any spinlock
> contention even for the "look up symlink" case.
>
> It also avoids the unnecessary aligned u64 for when we don't actually
> use cmpxchg at all.
>
> It's still one single patch, since I was working on lots of small
> cleanups. I think it's pretty close to done now (assuming your testing
> shows it performs fine - the powerpc numbers are promising, though),
> so I'll split it up into proper chunks rather than random commit
> points. But I'm done for today at least.
>
> NOTE NOTE NOTE! My test coverage really has been pretty pitiful. You
> may hit cases I didn't test. I think it should be *stable*, but maybe
> there's some other d_lock case that your tuned waiting hid, and that
> my "fastpath only for unlocked case" version ends up having problems
> with.
>

Following this thread with half an eye... Was that "unsigned" stuff
fixed (someone pointed to it).
How do you call that test-patch (subject)?
I would like to test it on my SNB ultrabook with your test-case script.

- Sedat -
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