On 2022-02-11 17:50:34 [+0100], Jason A. Donenfeld wrote:
> Hi Sebastian,
Hi Jason,
> > I *think* we could drop that "fast_pool !=
> > this_cpu_ptr(&irq_randomness)" check at the top since that cmpxchg will
> > save us and redo the loop. But if I remember correctly you worried about
> > fast_pool->pool being modified (which is only a corner case if we are on
> > the other CPU while the orig CPU is back again). Either way, it would be
> > random and we would not consume more entropy.
>
> No, we cannot, and "it's all random anyway so who cares if we corrupt
> things!" is not rigorous, as entropy may actually be thrown away as
> it's moved between words on each mix. If we're not running on the same
> CPU, one CPU can corrupt the other's view of fast pool before updating
> count. We must keep this.
Okay, I assumed something like that.
> > So if we have to keep this then please swap that migrate_disable() with
> > local_irq_disable(). Otherwise PeterZ will yell at me.
>
> Okay, I'll do that then, and then in the process get rid of the
> cmpxchg loop since it's no longer required.
So the only reason why we have that atomic_t is for rare case where run
on the remote CPU and need to remove the upper bit in the counter?
> > > if (unlikely(crng_init == 0)) {
> > > - if (fast_pool->count >= 64 &&
> > > + if (new_count >= 64 &&
> > > crng_fast_load(fast_pool->pool, sizeof(fast_pool->pool)) > 0) {
> > > - fast_pool->count = 0;
> > > + atomic_set(&fast_pool->count, 0);
> > > fast_pool->last = now;
> >
> > I'm fine if we keep this as is for now.
> > What do we do here vs RT? I suggested this
> > https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/commit/?id=a2d2d54409481aa23a3e11ab9559a843e36a79ec
> >
> > Is this doable?
>
> It might be, but last time I checked it seemed problematic. As I
> mentioned in an earlier thread, I'll take a look again at that next
> week after this patch here settles. Haven't forgotten.
Ah, cheers.
> v+1 coming up with irqs disabled.
>
> Jason
Sebastian
