A misunderstanding of the return value of 'dm_bit' has been causing a data

corruption bug in cmirror.  'dm_bit' is only ever used as a boolean operation
within LVM, but it can return a range of values.  If the bit is set, a power of
2 is returned.  If the bit is unset, 0 is returned.

'log_test_bit' (a function in the cluster mirror log daemon code) has switched
to using the dm bit operations in rhel6.  There are two places in the daemon
code where 'log_test_bit' is not used merely as a boolean, but rather the
return value is used as the return value for the log functions 'is_clean' and
'in_sync' - having assumed that 'dm_bit' was returning 0 or 1 only.

One place the 'in_sync' function is utilized is in 'dm_rh_get_state' - a
function that informs the mirroring code how to treat I/O and which devices to
read/write from.  'dm_rh_get_state' was checking if the return value of
'in_sync' was 1 to determine if the region was DM_RH_CLEAN.  Since 'dm_bit'
(and by extension 'log_test_bit' and 'in_sync') was returning powers of 2,
DM_RH_CLEAN was rarely being reported as it should have been.  Thinking the
region was out-of-sync, the mirroring code would write only to the primary
device.  When the primary device was failed, all of those writes were lost -
leaving the entire mirror corrupted.

WHATS_NEW

@@ -1,5 +1,6 @@
 Version 2.02.73 - 
 ================================
+  Fix data corruption bug in cluster mirrors.
   Require logical volume(s) to be explicitly named for lvconvert --merge.
   Avoid changing aligned pe_start as a side-effect of very verbose logging.
   Fix 'void*' arithmetic warnings in dbg_malloc.c.

daemons/cmirrord/functions.c

@@ -106,7 +106,7 @@ static DM_LIST_INIT(log_pending_list);
 
 static int log_test_bit(dm_bitset_t bs, int bit)
 {
-	return dm_bit(bs, bit);
+	return dm_bit(bs, bit) ? 1 : 0;
 }
 
 static void log_set_bit(struct log_c *lc, dm_bitset_t bs, int bit)