Date: Thu, 23 Aug 2007 14:40:37 -0500
From: Matt Mackall <mpm@selenic.com>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>, bunk@kernel.org,
       josh@kernel.org, linux-kernel@vger.kernel.org, mingo@elte.hu
Subject: Re: [PATCH] Make rcutorture RNG use temporal entropy
Message-ID: <20070823194037.GW30556@waste.org>
References: <20070816024904.GA5312@linux.vnet.ibm.com> <20070817115356.cb201e42.akpm@linux-foundation.org> <20070817200022.GH8464@linux.vnet.ibm.com> <20070823180658.GQ11166@waste.org> <20070823185830.GC8371@linux.vnet.ibm.com>
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On Thu, Aug 23, 2007 at 11:58:31AM -0700, Paul E. McKenney wrote:
> On Thu, Aug 23, 2007 at 01:06:58PM -0500, Matt Mackall wrote:
> > On Fri, Aug 17, 2007 at 01:00:22PM -0700, Paul E. McKenney wrote:
> > > On Fri, Aug 17, 2007 at 11:53:56AM -0700, Andrew Morton wrote:
> > > > On Wed, 15 Aug 2007 19:49:04 -0700
> > > > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> > > > 
> > > > > Repost of http://lkml.org/lkml/2007/8/10/472 made available by request.
> > > > > 
> > > > > The locking used by get_random_bytes() can conflict with the
> > > > > preempt_disable() and synchronize_sched() form of RCU.  This patch changes
> > > > > rcutorture's RNG to gather entropy from the new cpu_clock() interface
> > > > > (relying on interrupts, preemption, daemons, and rcutorture's reader
> > > > > thread's rock-bottom scheduling priority to provide useful entropy),
> > > > > and also adds and EXPORT_SYMBOL_GPL() to make that interface available
> > > > > to GPLed kernel modules such as rcutorture.
> > > > > 
> > > > > Passes several hours of rcutorture.
> > > > 
> > > > Please explain what "conflict with" means so that I can work out if
> > > > this is a needed-in-2.6.23 change, thanks.
> > > 
> > > Not needed in 2.6.23.  This change falls into the "preparation for -rt"
> > > category.  Also in the "don't unnecessarily eat entropy, leave some for
> > > the people needing crypographically secure randomness" category.
> > 
> > We've had several calls for a more fast and loose version of
> > get_random_bytes. Generalizing one of the cookie generation functions
> > is probably a good way to go.
> 
> Are you thinking in terms of secure_tcp_syn_cookie(), or did you have
> something else in mind?

Yes. Using a hash function rather than a trivial LFSR is preferable.
But pulling the guts out and giving it an n-bytes interface like
get_random_bytes().

-- 
Mathematics is the supreme nostalgia of our time.
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