flock() Race Condiiton?

[Christopher Milton]

--- Keary Suska <hierophant at pcisys.net> wrote:
> on 5/10/02 12:36 PM, cmiltonperl at yahoo.com purportedly said:
> >> Is anyone aware of a possible flock() race condition that could cause two
> >> processes vying for a lock to effectively lock each other out, and at the
> >> same time, lock out any access to that file? I am trying to debug a rather
> >> mysterious problem where a parallel processing system just hangs for some
> >> reason.
> > 
> > According to 'perldoc perlfunc':
> > Two potentially non-obvious but traditional flock
> > semantics are that it waits indefinitely until the
> > lock is granted, and that its locks merely
> > advisory.  Such discretionary locks are more
> > flexible, but offer fewer guarantees.  This means
> > that files locked with flock() may be modified by
> > programs that do not also use flock().
> 
> Yes, however, all of my processes use flock(). The issue is that for some
> reason, all of the processes seemingly hang at the same time, and all of
> them are accessing the same files. I did not mean to say that no process
> could access the file(s), that is, they can as long as they don't use
> flock().
> 
> Also, if there is no race condition, the first request for a lock should be
> granted, and all others should wait for the lock to be released. As far as I
> can tell, there is no reason that the lock shouldn't be released after being
> granted (no apparent infinite loop conditions, or premature subroutine
> exits). What I am wondering is if it is possible that two or more processes
> request a lock simultaneously, and both receive a current locked condition,
> so they wait, but the system establishes the lock, so all other processes
> are forced to wait as well, indefinitely.

The quote I posted says your processes will wait indefinitely until
the lock is _granted_. There seems to a possibility to my mind that
they could all be waiting for each other to relinquish the lock...
depending on the implementation. See 'perldoc perlfunc'.

> This is why I would like to be able to kick on the debugger while the
> process is running, so I can see what is going on.

Check out debug modules on CPAN. Read the docs through http://search.cpan.org/


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