Correct lock ASSERTs in vdev_label_read/write

The existing assertions in vdev_label_read() and vdev_label_write(),
testing which config locks are held, are incorrect. The assertions
test for locks which exceed what is required for safety.

Both vdev_label_{read,write}() are changed to assert SCL_STATE is held
as RW_READER or RW_WRITER. This is safe because:

Changes to the vdev tree occur under SCL_ALL as RW_WRITER, via
spa_vdev_enter() and spa_vdev_exit().

Changes to vdev state occur under SCL_STATE_ALL as RW_WRITER, via
spa_vdev_state_enter() and spa_vdev_state_exit().

Therefore, the new assertions guarantee that the vdev cannot change
out from under a zio, and I/O to a specified leaf vdev's label is
safe.

Furthermore, this is consistent with the SPA locking discussion in
spa_misc.c, "For any zio operation that takes an explicit vdev_t
argument ... zio_read_phys(), or zio_write_phys() ... SCL_STATE as
reader suffices."

Reviewed-by: Chunwei Chen <david.chen@osnexus.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Closes #5983

module/zfs/vdev_label.c

@@ -182,8 +182,9 @@ static void
 vdev_label_read(zio_t *zio, vdev_t *vd, int l, abd_t *buf, uint64_t offset,
     uint64_t size, zio_done_func_t *done, void *private, int flags)
 {
-	ASSERT(spa_config_held(zio->io_spa, SCL_STATE_ALL, RW_WRITER) ==
-	    SCL_STATE_ALL);
+	ASSERT(
+	    spa_config_held(zio->io_spa, SCL_STATE, RW_READER) == SCL_STATE ||
+	    spa_config_held(zio->io_spa, SCL_STATE, RW_WRITER) == SCL_STATE);
 	ASSERT(flags & ZIO_FLAG_CONFIG_WRITER);
 
 	zio_nowait(zio_read_phys(zio, vd,
@@ -196,10 +197,9 @@ static void
 vdev_label_write(zio_t *zio, vdev_t *vd, int l, abd_t *buf, uint64_t offset,
     uint64_t size, zio_done_func_t *done, void *private, int flags)
 {
-	ASSERT(spa_config_held(zio->io_spa, SCL_ALL, RW_WRITER) == SCL_ALL ||
-	    (spa_config_held(zio->io_spa, SCL_CONFIG | SCL_STATE, RW_READER) ==
-	    (SCL_CONFIG | SCL_STATE) &&
-	    dsl_pool_sync_context(spa_get_dsl(zio->io_spa))));
+	ASSERT(
+	    spa_config_held(zio->io_spa, SCL_STATE, RW_READER) == SCL_STATE ||
+	    spa_config_held(zio->io_spa, SCL_STATE, RW_WRITER) == SCL_STATE);
 	ASSERT(flags & ZIO_FLAG_CONFIG_WRITER);
 
 	zio_nowait(zio_write_phys(zio, vd,