ZTS: Fix create-o_ashift test case

The function that fills the uberblock ring buffer on every device label
has been reworked to avoid occasional failures caused by a race
condition that prevents 'zpool sync' from writing some uberblock
sequentially: this happens when the pool sync ioctl dispatch code calls
txg_wait_synced() while we're already waiting for a TXG to sync.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: loli10K <ezomori.nozomu@gmail.com>
Closes #6924 
Closes #6977

cmd/zdb/zdb.c

@@ -2788,10 +2788,6 @@ dump_label(const char *dev)
 		exit(1);
 	}
 
-	if (ioctl(fd, BLKFLSBUF) != 0)
-		(void) printf("failed to invalidate cache '%s' : %s\n", path,
-		    strerror(errno));
-
 	if (fstat64_blk(fd, &statbuf) != 0) {
 		(void) printf("failed to stat '%s': %s\n", path,
 		    strerror(errno));
@@ -2799,6 +2795,10 @@ dump_label(const char *dev)
 		exit(1);
 	}
 
+	if (S_ISBLK(statbuf.st_mode) && ioctl(fd, BLKFLSBUF) != 0)
+		(void) printf("failed to invalidate cache '%s' : %s\n", path,
+		    strerror(errno));
+
 	avl_create(&config_tree, cksum_record_compare,
 	    sizeof (cksum_record_t), offsetof(cksum_record_t, link));
 	avl_create(&uberblock_tree, cksum_record_compare,

tests/zfs-tests/tests/functional/cli_root/zpool_create/create-o_ashift.ksh

@@ -42,54 +42,47 @@
 
 verify_runnable "global"
 
-# See issue: https://github.com/zfsonlinux/zfs/issues/6924
-if is_linux; then
-	log_unsupported "Test case occasionally fails"
-fi
-
 function cleanup
 {
-	poolexists $TESTPOOL && destroy_pool $TESTPOOL
+	destroy_pool $TESTPOOL
 	log_must rm -f $disk
 }
 
 #
-# Commit the specified number of TXGs to the provided pool
+# Fill the uberblock ring in every <device> label: we do this by committing
-# We use 'zpool sync' here because we can't force it via sync(1) like on illumos
+# TXGs to the provided <pool> until every slot contains a valid uberblock.
-# $1 pool name
+# NOTE: We use 'zpool sync' here because we can't force it via sync(1) like on
-# $2 number of txg syncs
+# illumos
 #
-function txg_sync
+function write_device_uberblocks # <device> <pool>
 {
-	typeset pool=$1
+	typeset device=$1
-	typeset -i count=$2
+	typeset pool=$2
-	typeset -i i=0;
 
-	while [ $i -lt $count ]
+	while [ "$(zdb -quuul $device | grep -c 'invalid')" -ne 0 ]
 	do
-		log_must sync_pool $pool true
+		sync_pool $pool true
-		((i = i + 1))
 	done
 }
 
 #
-# Verify device $1 labels contains $2 valid uberblocks in every label
+# Verify every label on <device> contains <count> (valid) uberblocks
-# $1 device
-# $2 uberblocks count
 #
-function verify_device_uberblocks
+function verify_device_uberblocks # <device> <count>
 {
 	typeset device=$1
 	typeset ubcount=$2
 
 	zdb -quuul $device | egrep '^(\s+)?Uberblock' |
-	    egrep -v 'invalid$' | awk \
+	    awk -v ubcount=$ubcount 'BEGIN { count=0 } { uberblocks[$0]++; }
-	    -v ubcount=$ubcount '{ uberblocks[$0]++; }
+	    END {
-	    END { for (i in uberblocks) {
+	        for (i in uberblocks) {
-		count++;
+	            if (i ~ /invalid/) { continue; }
 	            if (uberblocks[i] != 4) { exit 1; }
+	            count++;
 	        }
-	    if (count != ubcount) { exit 1; } }'
+	        if (count != ubcount) { exit 1; }
+	    }'
 
 	return $?
 }
@@ -115,8 +108,7 @@ do
 		log_fail "Pool was created without setting ashift value to "\
 		    "$ashift (current = $pprop)"
 	fi
-	# force 128 txg sync to fill the uberblock ring
+	write_device_uberblocks $disk $TESTPOOL
-	txg_sync $TESTPOOL 128
 	verify_device_uberblocks $disk ${ubcount[$i]}
 	if [[ $? -ne 0 ]]
 	then