gfapi: Fix IO error caused when there is consecutive graph switches

This is part 2 of the fix, the part 1 can be found at:
http://review.gluster.org/#/c/14656/

Problem:
=======
Consider a race between, __glfs_active_subvol() and graph_setup().
Lets say @TIME T1:
fs->active_subvol = A
fs->next_subvol = B
__glfs_active_subvol()                //under lock fs->mutex
{
  ....
  new_subvol = fs->next_subvol       //which is B
  ....                               //Start migration from A to B
  __glfs_first_lookup(){
     ....
     unlock fs->mutex                //@TIME T2
     network fop
     lock fs->mutex
     ....
  }
  ....                                //migration continue on B
  fs->active_subvol = fs->next_subvol //which is C (explained below)
  ....
}

@Time T2, lets say in another thread, graph_setup() is called with C,
note that at T2, fs->mutex is unlocked.

graph_stup(C...)
{
  lock fs->mutex
  ....
  if (fs->next_subvol)                // which is B
      destroy subvol (fs->next_subvol)
  ....
  fs->next_subvol = C
  ....
  unlock fs->mutex
}

Thus at the end of this,
fs->old_subvol = A;
fs->active_subvol = C;
fs->next_subvol = NULL;
which is wrong, as B completed migration, but was destroyed by
graph_setup, and C never was migrated.

Solution:
=========
Any new graph can be in one of the 2 states:
- Picked for migration, migration in progress (fs->mip_subvol)
- Not picked so far for migration (fs->next_subvol)
graph_setup() updates fs->next_subvol only, __glfs_active_subvol()
moves fs->next_subvol to fs->mip_subvol and fs->next_subvol = NULL
atomically, and then once the migration is complete, make that the
fs->active_subvol

Change-Id: Ib6ff0565105c5eedb912a43da4017cd413243612
BUG: 1343038
Signed-off-by: Poornima G <pgurusid@redhat.com>
Reviewed-on: http://review.gluster.org/14722
NetBSD-regression: NetBSD Build System <jenkins@build.gluster.org>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>
Smoke: Gluster Build System <jenkins@build.gluster.org>
Reviewed-by: Raghavendra Talur <rtalur@redhat.com>
Reviewed-by: Rajesh Joseph <rjoseph@redhat.com>
Reviewed-by: Niels de Vos <ndevos@redhat.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/api/src/glfs-master.c b/api/src/glfs-master.c
index 9f11a6a0c9c..00a9c929a04 100644
--- a/api/src/glfs-master.c
+++ b/api/src/glfs-master.c
@@ -40,7 +40,8 @@ graph_setup (struct glfs *fs, glusterfs_graph_t *graph)
 	{
 		if (new_subvol->switched ||
 		    new_subvol == fs->active_subvol ||
-		    new_subvol == fs->next_subvol) {
+		    new_subvol == fs->next_subvol ||
+                    new_subvol == fs->mip_subvol) {
 			/* Spurious CHILD_UP event on old graph */
 			ret = 0;
 			goto unlock;