svn commit: r352938 - head/sys/arm/include

[Konstantin Belousov]

On Tue, Oct 01, 2019 at 07:39:00PM +0000, Ian Lepore wrote:
> Author: ian
> Date: Tue Oct  1 19:39:00 2019
> New Revision: 352938
> URL: https://svnweb.freebsd.org/changeset/base/352938
> 
> Log:
>   Add 8 and 16 bit versions of atomic_cmpset and atomic_fcmpset for arm.
>   
>   This adds 8 and 16 bit versions of the cmpset and fcmpset functions. Macros
>   are used to generate all the flavors from the same set of instructions; the
>   macro expansion handles the couple minor differences between each size
>   variation (generating ldrexb/ldrexh/ldrex for 8/16/32, etc).
>   
>   In addition to handling new sizes, the instruction sequences used for cmpset
>   and fcmpset are rewritten to be a bit shorter/faster, and the new sequence
>   will not return false when *dst==*old but the store-exclusive fails because
>   of concurrent writers. Instead, it just loops like ldrex/strex sequences
>   normally do until it gets a non-conflicted store. The manpage allows LL/SC
>   architectures to bogusly return false, but there's no reason to actually do
>   so, at least on arm.

The reason is to avoid nested loops.  The outer control for retry was the
initial design decision for fcmpset() comparing to cmpset().  casueword()
also started following this approach after the fixes for ll/sc looping
after the external control.