[PATCH] microoptimize locking primitives by avoiding unnecessary atomic ops

[Eric van Gyzen]

On 05/27/16 02:17 PM, Mateusz Guzik wrote:
> Hello there,
>
> quite some time ago I posted a trivial patch to locking primitives. What
> they do is the inline part tries an atomic op and if that fails the
> actual function is called, which immediately tries the same op.
>
> The obvious optimisation checks for the availability of the lock first.
>
> There concerns about the way it was done previously by relying on
> volatile behaving in a specific way.
>
> Later a simplified version was posted which should not have the concern,
> but the thread died.
>
> I refer you to https://lists.freebsd.org/pipermail/freebsd-current/2015-November/058100.html
> for simple benchmark results.
>
> I would like to get the patch in before 11 freeze.

This makes sense to me, and the patch looks good.

Please consider adding a comment to each location that explains why the
extra condition is tested before the atomic op.  Without such a comment,
I am concerned that your changes will be garbage collected later,
because the extra condition would seem superfluous to someone less
familiar with the code.

Eric