cluster/ec: Do not ref dictionary in lookup

Problem:
1) dict_for_each loops over the elements without any locks, so the members of
   the dictionary can be ref/unrefed while dict_for_each is executed by another
   thread leading to crashes.

Basically with distributed ec + disctributed replicate as cold, hot tiers. tier
sends a lookup which fails on ec. (By this time dict already contains ec
xattrs) After this lookup_everywhere code path is hit in tier which triggers
lookup on each of distribute's hash lookup but fails which leads to the cold,
hot dht's lookup_everywhere in two parallel epoll threads where in ec when it
tries to set trusted.ec.version/dirty/size as keys in the dictionary, the older
values against the same key get erased. While this erasing is going on if the
thread that is doing lookup on afr's subvolume accesses these keys either in
dict_copy_with_ref or client xlator trying to serialize, that can either lead
to crash or hang based on if the spin/mutex lock is called on invalid memory.

2) EC deletes GF_CONTENT_KEY from the dictionary, this may lead to extra reads
   in case of lookup-everwhere for tiered volumes.

Fix:
Do dict_copy_with_ref() for the lookup-dictionary.
This is avoiding the problem and is not actually fixing the 1st problem.
2nd problem will be fixed.

Change-Id: I5427aa14c48cb7572977d4de9a28c5ffff2b4b95
BUG: 1315560
Signed-off-by: Pranith Kumar K <pkarampu@redhat.com>
Reviewed-on: http://review.gluster.org/13680
Smoke: Gluster Build System <jenkins@build.gluster.com>
NetBSD-regression: NetBSD Build System <jenkins@build.gluster.org>
CentOS-regression: Gluster Build System <jenkins@build.gluster.com>
Reviewed-by: Xavier Hernandez <xhernandez@datalab.es>

1 files changed, 4 insertions, 7 deletions

diff --git a/xlators/cluster/ec/src/ec-generic.c b/xlators/cluster/ec/src/ec-generic.c
index 3f5856e7a86..375e49e695c 100644
--- a/xlators/cluster/ec/src/ec-generic.c
+++ b/xlators/cluster/ec/src/ec-generic.c
@@ -906,14 +906,11 @@ void ec_lookup(call_frame_t * frame, xlator_t * this, uintptr_t target,
         }
     }
     if (xdata != NULL) {
-        fop->xdata = dict_ref(xdata);
-        if (fop->xdata == NULL) {
-            gf_msg (this->name, GF_LOG_ERROR, 0,
-                    EC_MSG_DICT_REF_FAIL, "Failed to reference a "
-                                             "dictionary.");
-
+        fop->xdata = dict_copy_with_ref (xdata, NULL);
+        /* Do not log failures here as a memory problem would have already
+         * been logged by the corresponding alloc functions */
+        if (fop->xdata == NULL)
             goto out;
-        }
     }
 
     error = 0;