Monitors

[ambreenjava]

Moders Operating System By Andrew Tanenbaum

 

Synchronization within monitors uses condition variables and 2 special operations, WAIT and SIGNAL. A more general form of synchronization would be to have a single primitive, WAITUNTIL that had an arbitrary boolean predicate as parameter. Thus one could say

 

WAITUNTIL x

 

This approach though simple is not used. Why?

[ambreenjava]

One thing I guess thats wrong with WAITUNTIL is it wont block processes. The process will contine to check in loop, rather than sleeping and be awaked by the other process.

 

Lets say we write a producer consumer problem using WAITUNTIL

 

monitor ProducerConsumer

integer count;

 

procedure enter;

begin

if count = N then WAITUNTIL (count,EMPTY);

enter_item;

count := count + 1;

end

 

procedure remove;

begin

if count = 0 then WAITUNTIL(count,FULL);

remove_item;

count := count -1;

 

end;

 

end monitor;

Is there a race condition or a dead lock involved?

 

 

 

 

 

[rafi_dery]

ambreenjava, I agree with you.

 

Since the process won't block, but rather check for the condition, it will continue to occupy the monitor, avoiding other processes from entering the monitor. a deadlock occurs.

 

[ambreenjava]

O yeah, since only one process can be in a monitor at a time.