threads & mutex question (Rubyist book example)

[Suraj Kurapati]

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?

Thanks for your consideration.

 

[Robert Klemme]

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?

From what I see in your posting: yes.

Kind regards

robert

 

[David A. Black]

Hi --

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.

THE Ruby training with Black/Brown/McAnally
COMPLEAT Coming to Chicago area, June 18-19, 2010!
RUBYIST http://www.compleatrubyist.com

 

[Keith Johnston]

"Probably"? If by "probably" you mean "yes".

Won't do any "serious damage"? Since when is producing the wrong
answer not "serious" in programming?

You would be much better off making a frank admission the mistake. We
all know cognitive dissonance can be disorienting, but that's no
excuse to indulge it.

 

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