cluster/afr: Prevent split-brain when bricks are brought off and on in cyclic order

When the bricks are brought offline and then online in cyclic
order while writes are in progress on a file, thanks to inode
refresh in write txns, AFR will mostly fail the write attempt
when the only good copy is offline. However, there is still a
remote possibility that the file will run into split-brain if
the brick that has the lone good copy goes offline *after* the
inode refresh but *before* the write txn completes (I call it
in-flight split-brain in the patch for ease of reference),
requiring intervention from admin to resolve the split-brain
before the IO can resume normally on the file. To get around this,
the patch does the following things:
i) retains the dirty xattrs on the file
ii) avoids marking the last of the good copies as bad (or accused)
    in case it is the one to go down during the course of a write.
iii) fails that particular write with the appropriate errno.

This way, we still have one good copy left despite the split-brain situation
which when it is back online, will be chosen as source to do the heal.

Change-Id: I9ca634b026ac830b172bac076437cc3bf1ae7d8a
BUG: 1363721
Signed-off-by: Krutika Dhananjay <kdhananj@redhat.com>
Reviewed-on: http://review.gluster.org/15080
Tested-by: Pranith Kumar Karampuri <pkarampu@redhat.com>
Smoke: Gluster Build System <jenkins@build.gluster.org>
CentOS-regression: Gluster Build System <jenkins@build.gluster.org>
Reviewed-by: Ravishankar N <ravishankar@redhat.com>
Reviewed-by: Oleksandr Natalenko <oleksandr@natalenko.name>
NetBSD-regression: NetBSD Build System <jenkins@build.gluster.org>
Reviewed-by: Pranith Kumar Karampuri <pkarampu@redhat.com>

1 files changed, 24 insertions, 0 deletions

diff --git a/libglusterfs/src/common-utils.c b/libglusterfs/src/common-utils.c
index 3529ad7f897..311ed72d04c 100644
--- a/libglusterfs/src/common-utils.c
+++ b/libglusterfs/src/common-utils.c
@@ -4553,3 +4553,27 @@ gfid_to_ino (uuid_t gfid)
 
         return ino;
 }
+
+int
+gf_bits_count (uint64_t n)
+{
+        int val = 0;
+#ifdef _GNU_SOURCE
+        val = __builtin_popcountll (n);
+#else
+        n -= (n >> 1) & 0x5555555555555555ULL;
+        n = ((n >> 2) & 0x3333333333333333ULL) + (n & 0x3333333333333333ULL);
+        n = (n + (n >> 4)) & 0x0F0F0F0F0F0F0F0FULL;
+        n += n >> 8;
+        n += n >> 16;
+        n += n >> 32;
+        val = n & 0xFF;
+#endif
+        return val;
+}
+
+int
+gf_bits_index (uint64_t n)
+{
+    return ffsll(n) - 1;
+}