[Info-vax] counting-lock ?

[Michael Moroney]

Pierre <pierre.bru at gmail.com> writes:

>On Oct 25, 5:07 am, moro... at world.std.spaamtrap.com (Michael Moroney)
>wrote:

>> I think I have an algorithm that needs fleshing out.  You have a master
>> lock M as well as other locks, numbered 1 through n-1 (n=5 in this case).
>> A process tries to get the master lock in exclusive mode, waiting until it
>> can.  If it's waiting it normally means there are n processes running
>> already, but can also be transient.  If it gets the lock, it counts the
>> number of holders.  If it's less than n, it converts the master lock to
>> null mode, and gets an exclusive lock on one of the numbered locks. 
>> (this part needs work, but for now, assume it tries Lock 1 first, if it
>> can't get it immediately, it tries Lock 2, then lock 3 etc), then converts
>> the master lock to Null, allowing another process to get it.  If the
>> number of holders on the master lock = n, it does _not_ convert the lock
>> (thus blocking the next process) but instead requests access to all the
>> numbered locks. As long as the other processes hold these locks, the
>> process holding the master lock won't get any of them.  Once any of these
>> other processes exits, releasing one of the numbered locks, the process
>> holding the master lock in EX has that numbered lock, and converts the
>> master lock to NULL, allowing another process to run.

>your algorithm resemble mine but is easier to implement :) keeping the
>EX lock on the master lock make the life easier than converting it to
>NL after enqueing EX lock on numbered ones. and as other processes
>wanting to access the "multi-resource" are waiting behind the master
>lock and not behind the numbered locks, the DLM has less job to do.

I use the master lock to block processes beyond n, it is converted to NL 
if and only if at least one additional process is allowed to run.  
Otherwise, keeping it at EX keeps additional processes from running.