Messages in this thread | | | Subject | Re: [PATCH 01/10] random: make 'add_interrupt_randomness()' do something sane | From | Matt Mackall <> | Date | Mon, 09 Jul 2012 14:15:46 -0500 |
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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 <tytso@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.
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