Yo James! On Fri, 13 Jan 2023 13:43:38 -0800 (PST) James Browning <jamesb...@jamesb192.com> wrote:
> > On 01/13/2023 1:06 PM PST Hal Murray <halmur...@sonic.net> wrote:
> >
> >
> > If we make any changes to SHM, we should switch to a setup where
> > the memory is read only. The idea is to allow multiple readers.
> >
> > The trick to implementing that is to have 2 counters.
> > X and Y are initialized to the same value.
> > The writer bumps X, updates the data, then bumps Y.
> > The reader grabs Y, grabs the data, then grabs X.
> > If X and Y are the same the data is valid. If not, try again.
>
> I've heard you mention this before, and I specced it
> by calling them bookends instead and sticking them at
> opposite ends of a 4KiB page-sized struct.
How do you force 4kB page size? Or 4kB alignment? Maybe the
kernel is using hug pages?
On a modern CPU, you have no way to force what gets written first.
You used to me able to depend on the write order of memcpy(), but not
true for a while.
RGDS
GARY
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