[ruby-core:54403] [CommonRuby - Feature #8259] Atomic attributes accessors

Issue #8259 has been updated by headius (Charles Nutter).


Having some discussions with dbussink on IRC...

I think the most universally correct option is to have two different paths for reference CAS and value CAS, and rely upon users to choose the right one. As dbussink points out, the people who are going to use atomic ivars are much more likely to know what they're doing.

We don't want to have an additional method for ever single atomic ivar, so perhaps a parameter?

  class MyClass
    atomic_accessor :foo
  
    # generates CAS accessor similar to...
    def foo_cas(old, new, compare_value = false)
      ...
    end
  end
  
  mc = MyClass.new
  mc.foo = 5 # fixnum, so we'll want value CAS
  ...
  
  # true causes value comparison CAS like I implemented in atomic gem
  # false does reference CAS as normal, with variable behavior for some numeric values.
  mc.foo_cas(5, 6, true)

I can't imagine two separate methods getting approved but this form seems like it would be acceptable.
----------------------------------------
Feature #8259: Atomic attributes accessors
https://bugs.ruby-lang.org/issues/8259#change-38665

Author: funny_falcon (Yura Sokolov)
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 


=begin
Motivated by this gist ((<URL:https://gist.github.com/jstorimer/5298581>)) and atomic gem

I propose Class.attr_atomic which will add methods for atomic swap and CAS:

  class MyNode
    attr_accessor :item
    attr_atomic :successor

    def initialize(item, successor)
      @item = item
      @successor = successor
    end
  end
  node = MyNode.new(i, other_node)

  # attr_atomic ensures at least #{attr} reader method exists. May be, it should
  # be sure it does volatile access.
  node.successor

  # #{attr}_cas(old_value, new_value) do CAS: atomic compare and swap
  if node.successor_cas(other_node, new_node)
    print "there were no interleaving with other threads"
  end

  # #{attr}_swap atomically swaps value and returns old value.
  # It ensures that no other thread interleaves getting old value and setting
  # new one by cas (or other primitive if exists, like in Java 8)
  node.successor_swap(new_node)

It will be very simple for MRI cause of GIL, and it will use atomic primitives for
other implementations.

Note: both (({#{attr}_swap})) and (({#{attr}_cas})) should raise an error if instance variable were not explicitly set before.

Example for nonblocking queue: ((<URL:https://gist.github.com/funny-falcon/5370416>))

Something similar should be proposed for Structs. May be override same method as (({Struct.attr_atomic}))

Open question for reader:
should (({attr_atomic :my_attr})) ensure that #my_attr reader method exists?
Should it guarantee that (({#my_attr})) provides 'volatile' access?
May be, (({attr_reader :my_attr})) already ought to provide 'volatile' semantic?
May be, semantic of (({@my_attr})) should have volatile semantic (i doubt for that)?
=end



-- 
http://bugs.ruby-lang.org/