From: "David A. Black" <dblack@...>
Date: 2010-06-16T21:20:19+09:00
Subject: Re: threads & mutex question (Rubyist book example)

Hi --

On Sun, 6 Jun 2010, Suraj Kurapati wrote:

> Hi,
>
> I am concerned about the lack of mutual exlusion in code listing
> 14.2 "Chat server using TCPServer and threads" on page 430 in "The
> Well-Grounded Rubyist" book.  That code goes something like this:
>
>  chatters = []
>
>  while ...
>    Thread.new(...) do |c|
>      chatters.each { ... }
>      chatters.push c
>      chatters.each { ... }
>      chatters.delete c
>      chatters.each { ... }
>    end
>  end
>
> Multiple threads are spawned and each of them accesses the shared
> resource (the chatters array) directly without mutual exclusion.
>
> What happens when a thread inside chatters.each() gets descheduled
> and another thread proceeds to add or delete an object to chatters?
>
> Shouldn't there be a Mutex#synchronize call wrapping all access to
> the chatters array?

Probably. You're right that the code as it stands runs the risk of
modifying an array while iterating over it -- which, though it
probably wouldn't do any serious damage in this case, is (I think)
undefined as to its results in Ruby, and in any case is something I
don't (consciously :-) advocate doing.


David

-- 
David A. Black, Senior Developer, Cyrus Innovation Inc.

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