# Mutex question / request **URL:** \ **Category:** ruby-talk\ **Created:** [25 July 2004 21:06 UTC](https://rubytalk.org/t/mutex-question-request/12754 "2004-07-25T21:06:53Z")\ **Posts on this page:** 2\ **Page:** 1
**Author:** ![Brian\_Schroeder](https://avatars.discourse-cdn.com/v4/letter/b/b5a626/32.png) [@Brian\_Schroeder](https://rubytalk.org/u/Brian_Schroeder)\ **Post date:** [25 July 2004 21:06 UTC](https://rubytalk.org/t/mutex-question-request/12754/1 "2004-07-25T21:06:53Z")
Hello, playing around with threads I observed, that it is impossible to lock a mutex twice from within the same thread. Coming from a delphi background I'm used to have the possibility to enter a critical section as often as I like, as long as I'm in the same thread. That allows for some simpler programming, as I can for example allow the user to read from a protected accessor while he has aquired the lock for some bigger operation. To be more specific: The following is not working: === Test 1 === require 'thread' m = Mutex.new m.synchronize do   puts 'locked level 1'   m.synchronize do     puts 'level 2'   end end === EOF === I'd like to use it for this usecase: === usecase === class A   def initialize     @lock = Mutex.new   end   def x     result = ''     @lock.synchronize do       # Do something with result     end     result   end   def synchronize     @lock.synchronize do yield end   end end a = A.new a.synchronize do   # Do something with a   puts a.x   # Do some more with a end === EOF === I looked into the thread library and came up with some changes that should implement the behaviour I want. But my knowledge of ruby threads is quite limited, so I'd like your feedback on how good this is. Also I'm interested if you think that this is a good idea, and if it should be proposed for inclusion in the standard library? === MultiMutex === class MultiMutex \< Mutex   def initialize     @lockingthread = nil     @lockdepth = 0     super   end   def lock     Thread.critical = true     begin       if @lockingthread = Thread.current then         @lockdepth += 1       else         while (Thread.critical = true; @lockdepth \> 0)           @waiting.push Thread.current           Thread.stop         end         @lockdepth = 1       end     ensure       Thread.critical = false     end     self   end   def unlock     return @lockdepth unless @locked     Thread.critical = true     begin       if @lockdepth \> 1 then         @lockdepth -= 1         return @lockdepth       else         @lockingthread = nil         @lockdepth = 0         begin           t = @waiting.shift           t.wakeup if t         rescue ThreadError           retry         end       end     ensure       Thread.critical = false     end     begin       t.run if t     rescue ThreadError     end     @lockdepth   end end === EOF === > **···** > > -- > Brian Schröder > [http://www.brian-schroeder.de/](http://www.brian-schroeder.de/) ---
**Author:** ![Gennady1](https://avatars.discourse-cdn.com/v4/letter/g/a4c791/32.png) [@Gennady1](https://rubytalk.org/u/Gennady1)\ **Post date:** [26 July 2004 00:07 UTC](https://rubytalk.org/t/mutex-question-request/12754/2 "2004-07-26T00:07:06Z")
> Hello, > > playing around with threads I observed, that it is impossible to lock a > mutex twice from within the same thread. Coming from a delphi background Look at monitor.rb (comes in standard Ruby distribution). It has almost same API as Mutex, however it was designed to be re-enterable by the current owner. > I'm used to have the possibility to enter a critical section as often as I > like, as long as I'm in the same thread. That allows for some simpler > programming, as I can for example allow the user to read from a protected > accessor while he has aquired the lock for some bigger operation. > > To be more specific: > > The following is not working: > === Test 1 === > require 'thread' > > m = Mutex.new > > m.synchronize do >   puts 'locked level 1' >   m.synchronize do >     puts 'level 2' >   end > end > === EOF === > > I'd like to use it for this usecase: > > === usecase === > class A >   def initialize >     @lock = Mutex.new >   end > >   def x >     result = '' >     @lock.synchronize do >       # Do something with result >     end >     result >   end > >   def synchronize >     @lock.synchronize do yield end >   end > end > > a = A.new > a.synchronize do >   # Do something with a >   puts a.x >   # Do some more with a > end > === EOF === > > I looked into the thread library and came up with some changes that should > implement the behaviour I want. But my knowledge of ruby threads is quite > limited, so I'd like your feedback on how good this is. Also I'm > interested if you think that this is a good idea, and if it should be > proposed for inclusion in the standard library? > > === MultiMutex === > class MultiMutex \< Mutex >   def initialize >     @lockingthread = nil >     @lockdepth = 0 >     super >   end > >   def lock >     Thread.critical = true >     begin >       if @lockingthread = Thread.current then >         @lockdepth += 1 >       else >         while (Thread.critical = true; @lockdepth \> 0) >           @waiting.push Thread.current >           Thread.stop >         end >         @lockdepth = 1 >       end >     ensure >       Thread.critical = false >     end >     self >   end > >   def unlock >     return @lockdepth unless @locked >     Thread.critical = true >     begin >       if @lockdepth \> 1 then >         @lockdepth -= 1 >         return @lockdepth >       else >         @lockingthread = nil >         @lockdepth = 0 >         begin >           t = @waiting.shift >           t.wakeup if t >         rescue ThreadError >           retry >         end >       end >     ensure >       Thread.critical = false >     end >     begin >       t.run if t >     rescue ThreadError >     end >     @lockdepth >   end > end > === EOF === > > -- > Brian Schröder > [http://www.brian-schroeder.de/](http://www.brian-schroeder.de/) Sincerely, Gennady Bystritsky > **···** > > On Jul 25, 2004, at 2:06 PM, Brian Schroeder wrote: