linux-kernel - Re: Compilers and RCU readers: Once more unto the breach!

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Message-ID: <20150714004459.GA3090@linux.vnet.ibm.com>
Date:	Mon, 13 Jul 2015 17:44:59 -0700
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	linux-kernel@...r.kernel.org, c++std-parallel@...u.org,
	linux-arch@...r.kernel.org, gcc@....gnu.org
Cc:	Peter.Sewell@...cam.ac.uk, torvalds@...ux-foundation.org,
	Mark.Batty@...cam.ac.uk, peterz@...radead.org, will.deacon@....com,
	Ramana.Radhakrishnan@....com, dhowells@...hat.com,
	akpm@...ux-foundation.org, mingo@...nel.org, michaelw@...ibm.com
Subject: Re: Compilers and RCU readers: Once more unto the breach!

On Tue, May 19, 2015 at 05:55:10PM -0700, Paul E. McKenney wrote:
> Hello!
> 
> Following up on last year's discussion (https://lwn.net/Articles/586838/,
> https://lwn.net/Articles/588300/), I believe that we have a solution.  If
> I am wrong, I am sure you all will let me know, and in great detail.  ;-)
> 
> The key simplification is to "just say no" to RCU-protected array indexes:
> https://lkml.org/lkml/2015/5/12/827, as was suggested by several people.
> This simplification means that rcu_dereference (AKA memory_order_consume)
> need only return pointers.  This in ture avoids things like (x-x),
> (x*0), and (x%1) because if "x" is a pointer, these expressions either
> return non-pointers are compilation errors.  With a very few exceptions,
> dependency chains can lead -to- non-pointers, but cannot pass -through-
> them.
> 
> The result is that dependencies are carried only by operations for
> which the compiler cannot easily optimize the away those dependencies,
> these operations including simple assignment, integer offset (including
> indexing), dereferencing, casts, passing as a function argument, return
> values from functions and so on.  A complete list with commentary starts
> on page 28 of:
> 
> 	http://www.rdrop.com/users/paulmck/RCU/consume.2015.05.18a.pdf

And an update is available here:

	http://www.rdrop.com/users/paulmck/RCU/consume.2015.07.13a.pdf

Among other things, this update addresses the points about equality
comparisons introduced by the compiler, and outlines some of the
issues head-/tail-marked alternatives face with respect to abstraction.
The updated "Restricted Dependency Chains" section starts on page 28.

Thoughts?

							Thanx, Paul

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