[PATCH] microoptimize locking primitives by avoiding unnecessary atomic ops

Eric van Gyzen vangyzen at FreeBSD.org
Fri May 27 20:34:28 UTC 2016

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.


More information about the freebsd-current mailing list