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 -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
