zio_add_child: collapse into a single function

The child locking difference is simple enough to handle with a boolean.
The actual work is more involved, and it's easy to forget to change
things in both places when experimenting. Just collapse them.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #17382

Author: robn

include/sys/zio.h

@@ -621,7 +621,6 @@ extern zio_t *zio_walk_parents(zio_t *cio, zio_link_t **);
 extern zio_t *zio_walk_children(zio_t *pio, zio_link_t **);
 extern zio_t *zio_unique_parent(zio_t *cio);
 extern void zio_add_child(zio_t *pio, zio_t *cio);
-extern void zio_add_child_first(zio_t *pio, zio_t *cio);
 
 extern void *zio_buf_alloc(size_t size);
 extern void zio_buf_free(void *buf, size_t size);

module/zfs/zio.c

@@ -744,8 +744,8 @@ zio_unique_parent(zio_t *cio)
 	return (pio);
 }
 
-void
-zio_add_child(zio_t *pio, zio_t *cio)
+static void
+zio_add_child_impl(zio_t *pio, zio_t *cio, boolean_t first)
 {
 	/*
 	 * Logical I/Os can have logical, gang, or vdev children.
@@ -765,7 +765,11 @@ zio_add_child(zio_t *pio, zio_t *cio)
 	zl->zl_child = cio;
 
 	mutex_enter(&pio->io_lock);
-	mutex_enter(&cio->io_lock);
+
+	if (first)
+		ASSERT(list_is_empty(&cio->io_parent_list));
+	else
+		mutex_enter(&cio->io_lock);
 
 	ASSERT(pio->io_state[ZIO_WAIT_DONE] == 0);
 
@@ -776,44 +780,22 @@ zio_add_child(zio_t *pio, zio_t *cio)
 	list_insert_head(&pio->io_child_list, zl);
 	list_insert_head(&cio->io_parent_list, zl);
 
-	mutex_exit(&cio->io_lock);
+	if (!first)
+		mutex_exit(&cio->io_lock);
+
 	mutex_exit(&pio->io_lock);
 }
 
 void
-zio_add_child_first(zio_t *pio, zio_t *cio)
+zio_add_child(zio_t *pio, zio_t *cio)
 {
-	/*
-	 * Logical I/Os can have logical, gang, or vdev children.
-	 * Gang I/Os can have gang or vdev children.
-	 * Vdev I/Os can only have vdev children.
-	 * The following ASSERT captures all of these constraints.
-	 */
-	ASSERT3S(cio->io_child_type, <=, pio->io_child_type);
-
-	/* Parent should not have READY stage if child doesn't have it. */
-	IMPLY((cio->io_pipeline & ZIO_STAGE_READY) == 0 &&
-	    (cio->io_child_type != ZIO_CHILD_VDEV),
-	    (pio->io_pipeline & ZIO_STAGE_READY) == 0);
-
-	zio_link_t *zl = kmem_cache_alloc(zio_link_cache, KM_SLEEP);
-	zl->zl_parent = pio;
-	zl->zl_child = cio;
-
-	ASSERT(list_is_empty(&cio->io_parent_list));
-	list_insert_head(&cio->io_parent_list, zl);
-
-	mutex_enter(&pio->io_lock);
-
-	ASSERT(pio->io_state[ZIO_WAIT_DONE] == 0);
-
-	uint64_t *countp = pio->io_children[cio->io_child_type];
-	for (int w = 0; w < ZIO_WAIT_TYPES; w++)
-		countp[w] += !cio->io_state[w];
-
-	list_insert_head(&pio->io_child_list, zl);
+	zio_add_child_impl(pio, cio, B_FALSE);
+}
 
-	mutex_exit(&pio->io_lock);
+static void
+zio_add_child_first(zio_t *pio, zio_t *cio)
+{
+	zio_add_child_impl(pio, cio, B_TRUE);
 }
 
 static void