From: "headius (Charles Nutter)" <headius@...>
Date: 2013-04-17T03:57:01+09:00
Subject: [ruby-core:54363] [CommonRuby - Feature #8259] Atomic attributes accessors


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


The "while true" loop is there in order to re-check if the value is == after a change. My justification is that the only atomic part of this is the final CAS, but we want to pretend that the whole == + CAS is atomic; so this loops until either the current value is non-numeric, non-equal, or numeric + equal + has not changed since we last got it.

This pattern is used fairly often in the concurrency utilities on JVM for performing non-atomic logic surrounding an atomic update. Without the loop, an update that happens after the == check and before the CAS but which does not change the *value* of the currently-referenced object would fail. I don't think it should.
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Feature #8259: Atomic attributes accessors
https://bugs.ruby-lang.org/issues/8259#change-38638

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



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