glusterd: Fix race in pid file update

This patch only removes lines of code. For personal gratification, giving a
detailed explanation of what the problem was.

When glusterd spawns the local brick process, say when a reboot of the node
occurs,the glusterd_brick_start() and subsequently the
glusterd_volume_start_glusterfs() function gets called twice; from
glusterd_spawn_daemons() and glusterd_do_volume_quorum_action() respectively.
This causes a race, best described by a pseudo-code of current behaviour.

glusterd_volume_start_glusterfs()
{
       if(!brick process running) {

         step-a) reap pid file( i.e. unlink it)
         step-b) fork a brick process which creates and locks pid file and
                 binds the process to a socket.

        }

}

Time            Event
----            -----
T1              Call-1 arrives, completes step-a, starts step-b
T2              Call-2 arrives, enters step-a as Call-1's forked child is not
                yet running.
T3              Call-1's forked child is alive, creates pidfile and locks it,binds
                its address to a socket.
T4              Call-2 performs step-a; i.e.unlinks the pid file created by Call-1 !!
                (files can still be stil be unlinked despite a lockf on it)
T5              Call-2 does step-b, and the forked child process creates a *new* pid file
                with it's pid and locks this file.
T6              But Call-2's brick process is not able to bind to socket as it
                is already in use (courtesy T3) and hence exits (so no locks anymore on the pidfile).

Result:
-  Pid file now contains PID of an extinct brick process.
- `gluster volume status` shows this PID value. It also notices that there is no
lock held on pid file by the currently running brick process (created by Call-1)
and hence shows N/A for the online status.

Also, as a result of events at T4, "ls  -l /proc/<brick process PID>/fd/pidfile"
shows up as deleted.

Fix:
1.Do not unlink pid file. i.e. avoid step-a. Now at T5,Call-2's brick process
cannot obtain lock on pid file (Call-1's process still holds it) and exits.

2. Unrelated, but remove lock-unlock sequence in glusterfs_pidfile_setup()
which does not seem to be doing anything useful.

Change-Id: I18d3e577bb56f68d85d3e0d0bfd268e50ed4f038
BUG: 1035586
Signed-off-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-on: http://review.gluster.org/6786
Reviewed-by: Krishnan Parthasarathi <kparthas@redhat.com>
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>

1 files changed, 0 insertions, 12 deletions

diff --git a/xlators/mgmt/glusterd/src/glusterd-utils.c b/xlators/mgmt/glusterd/src/glusterd-utils.c
index 7c58e51addb..9fb2ab31e04 100644
--- a/xlators/mgmt/glusterd/src/glusterd-utils.c
+++ b/xlators/mgmt/glusterd/src/glusterd-utils.c
@@ -1307,16 +1307,6 @@ out:
         return ret;
 }
 
-/* Caller should ensure that brick process is not running*/
-static void
-_reap_brick_process (char *pidfile, char *brickpath)
-{
-        unlink (pidfile);
-        /* Brick process is not running and pmap may have an entry for it.*/
-        pmap_registry_remove (THIS, 0, brickpath,
-                              GF_PMAP_PORT_BRICKSERVER, NULL);
-}
-
 static int
 _mk_rundir_p (glusterd_volinfo_t *volinfo)
 {
@@ -1371,8 +1361,6 @@ glusterd_volume_start_glusterfs (glusterd_volinfo_t  *volinfo,
         if (gf_is_service_running (pidfile, NULL))
                 goto connect;
 
-        _reap_brick_process (pidfile, brickinfo->path);
-
         port = brickinfo->port;
         if (!port)
                 port = pmap_registry_alloc (THIS);