From: Markus <markus@...>
Date: 2004-10-09T00:54:48+09:00
Subject: Re: Problems with my first multithreaded Rupy programm

Try taking the cv.broadcast and the Thread.stop out of the mutex
syncronize block (e.g., let go before you tell the others to start).

-- Markus


On Fri, 2004-10-08 at 08:14, Tassilo Horn wrote:
> Hi,
> 
> I'm trying to get into Ruby and today it's time for
> multithreading. Therefore I wanted to write a program which does this:
> 
>         In a tearoom there are three thirsty drinkers. To cook a tea one
>         needs water, tea and a cup. Every one of these three drinkers
>         has one of those items with him.
>         Every time the waiter is called he puts two items on the
>         table. Now one drinker has all the items he needs and cooks his
>         tea. After that he calls the waiter and the fun begins again...
> 
> Ok, here's what I wrote so far and what produces this output:
> 
> ,----
> | (%:~/tmp/Teadrinker)- ./TeaRoom.rb
> | WAITER: Looking at the table!
> | WAITER: Oh, nothing on the table!
> | WAITER: Putting 1 and 2 on the table.
> | deadlock 0x4023b798: sleep:J(0x4022603c) (main) - ./TeaRoom.rb:94
> | deadlock 0x40225e70: sleep:-  - /usr/lib/ruby/1.8/thread.rb:96
> | deadlock 0x40225efc: sleep:-  - /usr/lib/ruby/1.8/thread.rb:96
> | deadlock 0x40225f88: sleep:-  - /usr/lib/ruby/1.8/thread.rb:96
> | deadlock 0x4022603c: sleep:-  - ./TeaRoom.rb:31
> | ./TeaRoom.rb:31: Thread(0x4022603c): deadlock (fatal)
> `----
> 
> <----- CODE ----->
> 
> #! /usr/bin/env ruby
> 
> require 'thread'
> 
> # For synchronisation
> mutex = Mutex.new
> cv = ConditionVariable.new
> 
> # items on the table:
> # 0: cup
> # 1: water
> # 2: tea
> table = []
> 
> waiter = Thread.new {
>   while true
>     mutex.synchronize {
>       puts "WAITER: Looking at the table!"
>       if table.length == 0
>         puts "WAITER: Oh, nothing on the table!"
>         sleep 2
>         while table[0] == table[1] do
>           table = [rand(3), rand(3)]
>         end
>         $stdout << "WAITER: Putting " << table[0] << " and " << table[1]
>   << " on the table.\n"
>       else
>         puts "WAITER: Why did you call me???"
>       end
>       # Call the waiting tea drinkers
>       cv.broadcast
>       Thread.stop
>     }
>   end
> }    
> 
> # Mike has a cup (0)
> mike = Thread.new { 
>   while true
>     mutex.synchronize {
>       cv.wait(mutex)
>       if table.include?(1) and table.include?(2)
>         puts "MIKE: Water and tea on table. I'll cook my tea."
>         table = []
>         sleep 3
>         puts "MIKE: Drinking my tea. Calling the waiter."
>         # Wake up the waiter-Thread
>         waiter.wakeup
>         sleep 3
>         puts "MIKE: Now I will read my newspaper."
>       else
>         puts "MIKE: Not what I need."
>       end
>     }
>   end
> }
> 
> # Al has water (1)
> al = Thread.new { 
>   while true
>     mutex.synchronize {
>       cv.wait(mutex)
>       if table.include?(0) and table.include?(2)
>         puts "AL: Cup and tea on table. I'll cook my tea."
>         table = []
>         sleep 3
>         puts "AL: Drinking my tea. Calling the waiter."
>         # Wake up the waiter-Thread
>         waiter.wakeup
>         sleep 3
>         puts "AL: Now I will read my newspaper."
>       else
>         puts "AL: Not what I need."
>       end
>     }
>   end
> }
>  
> # Ed has tea (2)
> ed = Thread.new { 
>   while true
>     mutex.synchronize {
>       cv.wait(mutex)
>       if table.include?(0) and table.include?(1)
>         puts "ED: Cup and water on table. I'll cook my tea."
>         table = []
>         sleep 3
>         puts "ED: Drinking my tea. Calling the waiter."
>         # Wake up the waiter-Thread
>         waiter.wakeup
>         sleep 3
>         puts "ED: Now I will read my newspaper."
>       else
>         puts "ED: Not what I need."
>       end
>     }
>   end
> }
> 
> waiter.join
> 
> <----- END OF CODE ----->
> 
> What's wrong with these lines?
> 
> Much thanks in advance,
> Tassilo