Issue #8259 has been updated by Vit Z.


Eric Wong wrote:
>  Right.  There's no way I will ever advocate a memory barrier of any
>  kind by default for reads or in-place updates.
>  
>  Unfortunately, changing capacity of an array or hash is tricky and
>  probably requires barriers for most or all cases (unless escape analysis
>  can elide barriers, but that's pie-in-the-sky territory).

My thesis and objection to adding "atomic" methods to `Array` or `Hash`
is that it would necessarily entail making them thread-safe as a whole. However
making them thread-safe on concurrent Ruby VMs is costly and carries a
performance penalty even for a single threaded usage. It will also be impossible
to undo (once Ruby declares `Hash` to be safe to use concurrently, there is
no going back on this).

I am all for addition of `ConcurrentHash` or `Concurrent::Array` (these
new data structures would have `cas` and `swap` methods), but for
performance reasons plain old `Hash` and `Array` should be kept
completely un-thread-safe.

----------------------------------------
Feature #8259: Atomic attributes accessors
https://bugs.ruby-lang.org/issues/8259#change-47156

* Author: Yura Sokolov
* Status: Open
* Priority: Normal
* Assignee: 
* Category: 
* Target version: Ruby 2.1.0
----------------------------------------
=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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