glusterd: big lock - a coarse-grained locking to prevent racesv3.4.0alpha3

There are primarily three lists that are part of glusterd process,
that are concurrently accessed. Namely, priv->volumes, priv->peers
and volinfo->bricks_list.

Big-lock approach
-----------------
WHAT IS IT?
Big lock is a coarse-grained lock which protects all three
lists, mentioned above, from racy access.

HOW DOES IT WORK?
At any given point in time, glusterd's thread(s) are in execution
_iff_ there is a preceding, inbound network event. Of course, the
sigwaiter thread and timer thread are exceptions.
A network event is an external trigger to glusterd, via the epoll
thread, in the form of POLLIN and POLLERR.
As long as we take the big-lock at all such entry points and yield
it when we are done, we are guaranteed that all the network events,
accessing the global lists, are serialised.

This amounts to holding the big lock at
- all the handlers of all the actors in glusterd. (POLLIN)
- all the cbks in glusterd. (POLLIN)
- rpc_notify (DISCONNECT event), if we access/modify
  one of the three lists. (POLLERR)

In the case of synctask'ized volume operations, we must remember that,
if we held the big lock for the entire duration of the handler,
we may block other non-synctask rpc actors from executing.
For eg, volume-start would block in PMAP SIGNIN, if done incorrectly.
To prevent this, we need to yield the big lock, when we yield the
synctask, and reacquire on waking up of the synctask.

BUG: 948686
Change-Id: I429832f1fed67bcac0813403d58346558a403ce9
Signed-off-by: Krishnan Parthasarathi <kparthas@redhat.com>
Reviewed-on: http://review.gluster.org/4835
Tested-by: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Vijay Bellur <vbellur@redhat.com>

1 files changed, 11 insertions, 0 deletions

diff --git a/xlators/mgmt/glusterd/src/glusterd.h b/xlators/mgmt/glusterd/src/glusterd.h
index 41b74a684dd..a4f2d23ac2c 100644
--- a/xlators/mgmt/glusterd/src/glusterd.h
+++ b/xlators/mgmt/glusterd/src/glusterd.h
@@ -152,6 +152,7 @@ typedef struct {
         xlator_t       *xl;  /* Should be set to 'THIS' before creating thread */
         gf_boolean_t   pending_quorum_action;
         dict_t             *opts;
+        synclock_t      big_lock;
 } glusterd_conf_t;
 
 
@@ -423,6 +424,16 @@ __glusterd_uuid()
 	return &priv->uuid[0];
 }
 
+int glusterd_big_locked_notify (struct rpc_clnt *rpc, void *mydata,
+                                rpc_clnt_event_t event,
+                                void *data, rpc_clnt_notify_t notify_fn);
+
+int
+glusterd_big_locked_cbk (struct rpc_req *req, struct iovec *iov,
+                         int count, void *myframe, fop_cbk_fn_t fn);
+
+int glusterd_big_locked_handler (rpcsvc_request_t *req, rpcsvc_actor actor_fn);
+
 int32_t
 glusterd_brick_from_brickinfo (glusterd_brickinfo_t *brickinfo,
                                char **new_brick);