linux-kernel - Re: PATCH: 2.6.26-rc8: Fix IRQF_DISABLED for shared interrupts

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Message-ID: <20080630142822.GA22984@khazad-dum.debian.net>
Date:	Mon, 30 Jun 2008 11:28:22 -0300
From:	Henrique de Moraes Holschuh <hmh@....eng.br>
To:	Stefan Becker <Stefan.Becker@...ia.com>
Cc:	ext David Brownell <david-b@...bell.net>,
	linux-kernel@...r.kernel.org, linux-usb@...r.kernel.org,
	ext Alan Stern <stern@...land.harvard.edu>
Subject: Re: PATCH: 2.6.26-rc8: Fix IRQF_DISABLED for shared interrupts

On Mon, 30 Jun 2008, Stefan Becker wrote:
> Good question. What happens when you mix a random and a not-so-random  
> source: does the result have an as good random quality as the original  

It becomes unusable.  In fact, I find it likely that shared IRQs are not
safe for random data gathering at all in general. IRQs are not in fact
completely random, they just (sometimes) have a very small ammount of
randomness in them.  And the combined result you see in a shared IRQ line
could be correlated or "get somewhat more correlated" because of the sharing
(they go over the same BUS -> one can delay the other, etc)... and that
correlation might be dangerous.

IMO, the safe thing to do is to block shared IRQs from being used as random
sources.

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh
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