Comment 0 for bug 1160490

Ubuntu 12.04.2
ifupdown 0.7~beta2ubuntu8

Symptom: Every so often, /etc/init/network-interface.conf fails to bring up the loopback interface.

> Mar 25 16:39:37 XXXXXXXX kernel: [ 28.793922] init: network-interface (lo) pre-start process (1079) terminated with status 1

/var/log/upstart/network-interface-lo shows:

> ifup: failed to overwrite statefile /run/network/ifstate: No such file or directory

Relevant section of the ifup sources, in update_state():

        if (rename (tmpstatefile, statefile)) {
                fprintf(stderr,
                        "%s: failed to overwrite statefile %s: %s\n",
                        argv0, statefile, strerror(errno));
                exit (1);
        }

update_state() opens the statefile, gets a F_SETLKW lock on it, opens a tmpfile, filters the contents of the statefile into the tmpfile, closes the tmpfile, then renames the tempfile over the statefile.

Once the rename() happens in one instance of ifup, any other blocked instances are waiting around to lock a file that no longer exists in the filesystem. Overlap enough instances of ifup just right and you have them all locking different copies of statefile, which then doesn't prevent any of them from rename()ing tmpstatefile out from underneath the others, thus causing their own rename()s to fail with ENOENT.

Example:

Process A starts, opens statefile.
Process A locks statefile.
Process B starts, opens statefile.
Process B waits for lock on statefile.
Process A renames tmpstatefile to statefile and exits.
Process B acquires lock on *outdated* statefile FILE pointer.
Process C starts, opens current statefile (written by Process A).
Process C locks current statefile.
** Two ifups now have locks **
Process B renames tmpstatefile to statefile and exits.
Process C tries to rename tmpstatefile, fails because tmpstatefile has already been renamed out from under it by Process B.

NOTE: Since Process B was operating on an outdated statefile, it has also stomped over any changes made by Process A, so simply making the tmpstatefile process-specific to avoid rename()ing out from under each other won't help.