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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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