Subj : thread control by mutex, predicates and conditions puzzle
To   : comp.programming.threads
From : roland
Date : Sun Jan 02 2005 04:03 pm

I am seeking for help on a problem that
puzzles me for a while yet.


Given:
Multiple threads t1, t2, ...
A variable a,
and N variables v1, ... vN.

a is holding a state that controls global
thread (t1) behaviour, while v1,...vN deal with
interthread communication issues. E.g. a
contains a flag requesting the thread to abort
whatever it is doing and to restart.
v1..vN control buffers and the like.

My question now is how to protect the variables
correctly by mutexes and how to assign
predicates and conditions?

One obvious solution to me, is to have exactly
one mutex, one predicate and one condition.

So the next question is: Is this the only
valid solution possible?

I am afraid yes, but I did not yet manage
to prove this formally.

Some thoughts:
Assume v1 is evaluated by predicate p1, v2 by p2, ....
Variable a is evaluated by predicate pa.

Further p1 has condition c1, p2 has c2, ... and pa has ca.

Since evaluating any of predicates p1,..pN does not
reveal information about pa I need to modify my
predicates as following:
p1' = pa || p1
p2' = pa || p2
....
pN' = pa || pN
and associated conditions c1' ... cN'.

Now since a condition can be bound at most to a single
mutex, when performing a wait on any of c1,...cN I am
forced to use this mutex also for protection of a.

If it were different in order to change state of a, an
external thread would need to know on which of the
c1',...cN' the thread is currently blocked. It could then
lock a hypothetical mutex and change a. However this
information is not available. The only remedy is to have
a single mutex.

There still are mutiple predicates p1',... and conditions
c1',...
What happens when I change the state of a?
I lock the mutex, change a and then I need to
signal _all_ c1', ... cN'. (Since I do not know from
outside on which ci' the thread is currently blocking,
if at all.)

Since this is impractical from a source code
maintanance perspective (would need to add a signaling
function whenever a new predicate is added) I am stuck
with exactly one predicate condition pair.

If this result indeed is true I am asking if there is
another pattern that helps me solve this kind of problem.
If it is not true, and I am overlooking something important
I would kindly ask to point me to where I am wrong.

Thank you for your attention.

Roland

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