On Fri, 2012-07-06 at 12:52 -0400, Theodore Ts'o wrote:
> On Fri, Jul 06, 2012 at 09:24:00AM -0700, Linus Torvalds wrote:
> > On Fri, Jul 6, 2012 at 6:01 AM, Theodore Ts'o <ty...@mit.edu> wrote:
> > > What in the world is "fast count"? I've grepped for it,
> > > and I can't find it.
> >
> > It's your own fast-pool counter that Matt was talking about.
>
> When he said "check it against HZ", it confused me, since there's no
> way to compare it against HZ. But yes, I can certainly not give any
> credit for entropy if __IRQF_TIMER is set, or keep track of whether
> the previous interrupt had __IRQF_TIMER set in its descriptor. That's
> simple enough.
>
> I thought he was saying there was some way to distinguish between
> interrupts triggered by the clock interrupt versus other devices on
> the same irq channel --- and I couldn't figure out any to do that in
> an architecture independent way.
Sorry.. offline for the weekend.
Let me restate:
- on some architectures, we will call into the RNG on timer interrupts
- this is generally desirable, as most time sources are asynchronous to
sched_clock() and thus a source of entropy
- we also want to keep conditional checks like IRQF_TIMER off the fast
path
- but on systems where the timer interrupt is the primary time source,
we may get effectively no entropy when the system is quiescent
- so we should check the fast pool count against HZ before crediting
- but even then, we still should mix the fast pool
Something like:
add_some_randomness(...) /* always mix */
if (fast_pool->count > HZ) {
fast_pool->count = 0;
credit_entropy_pool(...); /* only credit when we've got > HZ events */
}
That should be safe on all systems.
--
Mathematics is the supreme nostalgia of our time.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
