Date: Mon, 23 Feb 2015 18:09:02 -0800 From: John-Mark Gurney <jmg@funkthat.com> To: Konstantin Belousov <kostikbel@gmail.com> Cc: arch@freebsd.org Subject: Re: locks and kernel randomness... Message-ID: <20150224020902.GZ46794@funkthat.com> In-Reply-To: <20150224015721.GT74514@kib.kiev.ua> References: <20150224012026.GY46794@funkthat.com> <20150224015721.GT74514@kib.kiev.ua>
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Konstantin Belousov wrote this message on Tue, Feb 24, 2015 at 03:57 +0200: > On Mon, Feb 23, 2015 at 05:20:26PM -0800, John-Mark Gurney wrote: > > I'm working on simplifying kernel randomness interfaces. I would like > > to get read of all weak random generators, and this means replacing > > read_random and random(9) w/ effectively arc4rand(9) (to be replaced > > by ChaCha or Keccak in the future). > > > > The issue is that random(9) is called from any number of contexts, such > > as the scheduler. This makes locking a bit more interesting. Currently, > > both arc4rand(9) and yarrow/fortuna use a default mtx lock to protect > > their state. This obviously isn't compatible w/ the scheduler, and > > possibly other calling contexts. > > > > I have a patch[1] that unifies the random interface. It converts a few > > of the locks from mtx default to mtx spin to deal w/ this. > This is definitely an overkill. The rebalancing minor use of randomness > absolutely does not require cryptographical-strenght randomness to > select a moment to rebalance thread queue. Imposing the spin lock on > the whole random machinery just to allow the same random gathering code > to be used for balance_ticks is detriment to the system responsivness. > Scheduler is fine even with congruential generators, as you could see in > the cpu_search(), look for the '69069'. Then why doesn't it use this then? This is exactly why I don't want random to be a congruential generator... If you're so sure that you don't need cryptographic secure and that it's a performance benefit to do so, then you're free to roll your own, but almost all of the time, code won't meet both requirements. I haven't audited all the places where random is currently being called that might require not sleeping. The scheduler happens to be the first one I ran into... > Please do not enforce yet another spinlock for the system. I sent this email asking for help for how to avoid a spin lock. I'd appreciate if you could suggest how to improve my patch. Thanks. -- John-Mark Gurney Voice: +1 415 225 5579 "All that I will do, has been done, All that I have, has not."
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