Re: [PATCH v7 2/6] MCS Lock: optimizations and extra comments

From: Tim Chen
Date: Mon Jan 20 2014 - 14:11:34 EST

On Mon, 2014-01-20 at 14:58 +0100, Peter Zijlstra wrote:
> On Thu, Jan 16, 2014 at 04:08:20PM -0800, Tim Chen wrote:
> > Remove unnecessary operation and make the cmpxchg(lock, node, NULL) == node
> > check in mcs_spin_unlock() likely() as it is likely that a race did not occur
> > most of the time.
> It might be good to describe why the node->locked=1 is thought
> unnecessary. I concur it is, but upon reading this changelog I was left
> wondering and had to go read the code and run through the logic to
> convince myself.
> Having done so, I'm now wondering if we think so for the same reason --
> although I'm fairly sure we are.
> The argument goes like: everybody only looks at his own ->locked value,
> therefore the only one possibly interested in node->locked is the lock
> owner. However the lock owner doesn't care what's in it, it simply
> assumes its 1 but really doesn't care one way or another.

Yes, it is done for the same reason you mentioned. I'll update
the comments better to reflect this.

> That said, a possible DEBUG mode might want to actually set it, validate
> that all other linked nodes are 0 and upon unlock verify the same before
> flipping next->locked to 1.

I'll leave a comment to indicate this. If we need a DEBUG mode later,
we can come back to add this easily.



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