Re: [PATCH] random: ensure mix_interrupt_randomness() is consistent

[Jason A. Donenfeld]

Sorry, missed this in your last email:

On Fri, Feb 11, 2022 at 9:16 AM Sebastian Andrzej Siewior
<bigeasy@xxxxxxxxxxxxx> wrote:
> > +     do {
> > +             count_snapshot = (unsigned int)atomic_read(&fast_pool->count);
> > +             for (i = 0; i < ARRAY_SIZE(pool); ++i)
> > +                     pool[i] = READ_ONCE(fast_pool->pool_long[i]);
>
> Why do you avoid memcpy()? Since it is a small memcpy, I'm sure the
> compile will inline the register moves.

Because the compiler will otherwise reorder it to be after the two
counter reads. I checked. And a barrier() is too heavy as it flushes
the writes to the stack instead of keeping them read into registers.
READ_ONCE() is the exact semantics we care about here.

Jason