Thanks for fixing the code but I guess you are missing the point. My contention in an earlier message was that this piece of code by BrowserUk is not thread safe: 

sub safe_set {
   my $k = shift;
   my $v :shared = shift;

   lock $$h{ $k };
   $h{$k} = \$v;
}

[download]

My reasoning of it was as follows: 


TIMESTEP 0:

    Say, $h{$k} points to v1 
    Thread T0: Holding lock on v1
    Thread T1: Waiting on Lock on v1 

TIMESTEP 1:

    T0 changes $h{$k} = \$v2, releases lock on $v1

TIMESTAMP 2: (Thread T2 makes an entrance)

    Thread T2: Acquires Lock on v2, proceeds
    Thread T1: Acquires Lock on v1, proceeds
    <Race-around condition>

[download]

In other words, the problem with the above safe_set implementation is that it creates a new $v so threads could be waiting on different "copies" of $v, thereby causing a race condition.

To test out safe_set implementation, I made the hash %h to have a single key and tried updates using several threads. Notice that I copy and then add one to it. If there is a race condition the value of variable is surely affected. The code did not run at all and failed giving out a MUTEX_LOCK error: So, the real question here is if my testing code (given below) really points to a problem with the safe_set function. The way you have implemented it basically changes the flavor of safe_set, which of course runs.. I hope you are able to see my point here. 

#! perl -slw

use strict;
use threads;
use threads::shared;

our %h : shared;

# Setting up just a single row to increase chances 
# a race condition
our $k = 'AAA';
our $val : shared = 0;
our $THREADS = 50;
our $iter = 50;
$h{$k} = \$val;

# Safe_set similar to BrowserUk 
sub safe_set {
  my $v :shared;
  # Critical Section
  {
     lock ${$h{$k}};
     $v = ${$h{$k}} + 1;
     $h{$k} = \$v;
  }
}

# Keep locking to increment $$h{$k}
sub test_safe_set {
  for (my $i = 0; $i < $iter; ++$i) {
     safe_set();
  }
}

my @pool = map{
      threads->create(\&test_safe_set)
    } 1 .. $THREADS;

$_->join for @pool;

warn ${$h{$k}}, "failed\n" if ${$h{$k}} != $THREADS * $iter;

[download]

In reply to Re^6: Avoid Locking Entire Hashes by jagan_1234
in thread Avoid Locking Entire Hashes by jagan_1234

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