linux-kernel - Re: [PATCH 02/10] random: use lockless techniques when mixing entropy pools

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Message-ID: <20120705194742.GG31225@thunk.org>
Date:	Thu, 5 Jul 2012 15:47:42 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Matt Mackall <mpm@...enic.com>
Cc:	Linux Kernel Developers List <linux-kernel@...r.kernel.org>,
	torvalds@...ux-foundation.org, w@....eu, ewust@...ch.edu,
	zakir@...ch.edu, greg@...ah.com, nadiah@...ucsd.edu,
	jhalderm@...ch.edu, tglx@...utronix.de, davem@...emloft.net
Subject: Re: [PATCH 02/10] random: use lockless techniques when mixing
 entropy pools

On Thu, Jul 05, 2012 at 02:10:12PM -0500, Matt Mackall wrote:
> 
> I don't think this will work correctly. It's important that simultaneous
> _readers_ of the state get different results. Otherwise, you can get
> things like duplicate UUIDs generated on different cores, something
> that's been observed in the field(!). I thought I added a comment to
> that effect some years back, but I guess not.

That's easy to fix; we just need to take a spinlock on the extract
side.  The main issue is that Thomas very much doesn't want us to take
a spinlock in the interrupt handler codepath.  But if we take a
spinlock while extracting, that should take care of this concern.

	       		   	       	    - Ted
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