Re: [PATCH 1/1] Fix: trace sched switch start/stop racy updates

[Linus Torvalds]

On Sat, Aug 17, 2019 at 1:28 AM Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> 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