On Sat, Aug 17, 2019 at 1:28 AM Linus Torvalds
<torva...@linux-foundation.org> wrote:
>
> unsigned int bits = some_global_value;
> ...test different bits in in 'bits' ...
>
> can easily cause multiple reads (particularly on a CPU that has a
> "test bits in memory" instruction and a lack of registers.
>
> So then doing it as
>
> unsigned int bits = READ_ONCE(some_global_value);
> .. test different bits in 'bits'...
Side note: this is likely the best example of actual WRITE_ONCE() use
too: if you have that global value with multiple bits that actually
have some interdependencies, then doing
some_global_value = some_complex_expression();
might be reasonably compiled to do several rmw instructions to update
'some_global_value'
So then
WRITE_ONCE(some_global_value, some_complex_expression());
really can be a good thing - it clearly just writes things once, and
it also documents the whole "write one or the other" value, not some
mid-way one, when you then look at the READ_ONCE() thing.
But I'm seeing a lot of WRITE_ONCE(x, constantvalue) kind of things
and don't seem to find a lot of reason to think that they are any
inherently better than "x = constantvalue".
(In contrast, using "smp_store_release(flag, true)" has inherent
value, because it actually implies a memory barrier wrt previous
writes, in ways that WRITE_ONCE() or a direct assignment does not.)
Ok, enough blathering. I think I've made my point.
Linus
