linux-kernel - Re: [RFC][PATCH] module: Optimize __module

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Message-ID: <20150302165811.GP15405@linux.vnet.ibm.com>
Date:	Mon, 2 Mar 2015 08:58:12 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Andi Kleen <ak@...ux.intel.com>,
	Andi Kleen <andi@...stfloor.org>, x86@...nel.org,
	linux-kernel@...r.kernel.org, oleg@...hat.com,
	rusty@...tcorp.com.au, mingo@...nel.org
Subject: Re: [RFC][PATCH] module: Optimize __module_address() using a latched
 RB-tree

On Mon, Mar 02, 2015 at 10:24:40AM +0100, Peter Zijlstra wrote:
> On Sat, Feb 28, 2015 at 03:32:03PM -0800, Paul E. McKenney wrote:
> > Whew!
> > 
> > Though otherwise whatever you were doing would have been pretty cool
> > and fun to learn about.  ;-)
> 
> So I think I can do that; where readers and writers are fully separated,
> but it requires:
> 
>  - tripple latch
>  - copy operator
>  - nested RCU
> 
> And the result would be horribly expensive (mostly due to the copy
> operator on dynamic data structures) on the update side, which severely
> limits the applicability of the scheme.

True enough, if you have a single pointer to an RCU-protected data
structure, you can update anything in any way by doing a deep copy of
the original, updating, and swapping pointers.  And what is a little
copy overhead among friends?  ;-)

							Thanx, Paul

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