On 6/23/07, Michael Scondo <[EMAIL PROTECTED]> wrote:
sub thread1{
print "1\n";
lock $x;
print "locked x: 1\n";
cond_wait $x;
Can't get past here until $x is signalled by another thread, and
unlocked by all other threads.
print "thread1\n";
lock $y;
cond_signal $y;
print "t1\n";
}
sub thread2{
sleep 1;
lock $y;
# {
lock $x;
print "locked x: 2\n";
cond_signal $x;
# }
If the curly braces of that naked block were present, the lock on $x
would be released here, at the end of that scope. But it's still
locked, because cond_signal didn't unlock it either. (Did you think it
would?)
print "thread2\n";
sleep 1;
cond_wait $y;
Can't get past here until $y is signalled by another thread, and
unlocked by all other threads.
print "t2\n";
}
As written, this is the end-of-scope for the lock on $x (as well as
the one on $y). But unless some third thread can rescue these first
two, they seem to be deadlocked, with the first waiting for $x to be
unlocked and the second waiting for $y to be signalled.
As you discovered, using the inner braces will release the lock on $x
soon enough to avoid the deadlock.
By the way, thanks greatly for the detailed, self-contained test case;
it showed exactly what you were doing.
Have fun with Perl!
--Tom Phoenix
Stonehenge Perl Training
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