On Apr 11, 2011, at 7:25 PM, Tom Lane wrote: > Robert Haas <robertmh...@gmail.com> writes: >> On Sun, Apr 10, 2011 at 5:03 PM, A.M. <age...@themactionfaction.com> wrote: >>> To ensure that no two postmasters can startup in the same data directory, I >>> use fcntl range locking on the data directory lock file, which also works >>> properly on (properly configured) NFS volumes. Whenever a postmaster or >>> postmaster child starts, it acquires a read (non-exclusive) lock on the >>> data directory's lock file. When a new postmaster starts, it queries if >>> anything would block a write (exclusive) lock on the lock file which >>> returns a lock-holding PID in the case when other postgresql processes are >>> running. > >> This seems a lot leakier than what we do now (imagine, for example, >> shared storage) and I'm not sure what the advantage is. > > BTW, the above-described solution flat out doesn't work anyway, because > it has a race condition. Postmaster children have to reacquire the lock > after forking, because fcntl locks aren't inherited during fork(). And > that means you can't tell whether there's a just-started backend that > hasn't yet acquired the lock. It's really critical for our purposes > that SysV shmem segments are inherited at fork() and so there's no > window where a just-forked backend isn't visible to somebody checking > the state of the shmem segment.
Then you haven't looked at my patch because I address this race condition by ensuring that a lock-holding violator is the postmaster or a postmaster child. If such as condition is detected, the child exits immediately without touching the shared memory. POSIX shmem is inherited via file descriptors. Cheers, M -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
