Merge tag 'for-upstream' of https://repo.or.cz/qemu/kevin into staging

Block layer patches

- Deadlock fixes: Do not drain while holding the graph lock
- qdev-properties-system: Fix assertion failure in set_drive_helper()
- iotests: fix 240

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* tag 'for-upstream' of https://repo.or.cz/qemu/kevin: (24 commits)
  hw/core/qdev-properties-system: Add missing return in set_drive_helper()
  iotests: fix 240
  block/io: remove duplicate GLOBAL_STATE_CODE() in bdrv_do_drained_end()
  iotests/graph-changes-while-io: add test case with removal of lower snapshot
  iotests/graph-changes-while-io: remove image file after test
  block: mark bdrv_drained_begin() and friends as GRAPH_UNLOCKED
  blockdev: drain while unlocked in external_snapshot_action()
  blockdev: drain while unlocked in internal_snapshot_action()
  block: move drain outside of quorum_del_child()
  block: move drain outside of bdrv_root_unref_child()
  block: move drain outside of quorum_add_child()
  block: move drain outside of bdrv_attach_child()
  block: move drain outside of bdrv_root_attach_child()
  block: move drain outside of bdrv_set_backing_hd_drained()
  block: move drain outside of bdrv_attach_child_common(_abort)()
  block: move drain outside of bdrv_try_change_aio_context()
  block: move drain outside of bdrv_change_aio_context() and mark GRAPH_RDLOCK
  block: mark bdrv_child_change_aio_context() GRAPH_RDLOCK
  block: mark change_aio_ctx() callback and instances as GRAPH_RDLOCK(_PTR)
  block: mark bdrv_parent_change_aio_context() GRAPH_RDLOCK
  ...

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

1 files changed, 157 insertions, 78 deletions

diff --git a/block.c b/block.c
index f222e1a50a..bfd4340b24 100644
--- a/block.c
+++ b/block.c
@@ -106,9 +106,9 @@ static void bdrv_reopen_abort(BDRVReopenState *reopen_state);
 
 static bool bdrv_backing_overridden(BlockDriverState *bs);
 
-static bool bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
-                                    GHashTable *visited, Transaction *tran,
-                                    Error **errp);
+static bool GRAPH_RDLOCK
+bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
+                        GHashTable *visited, Transaction *tran, Error **errp);
 
 /* If non-zero, use only whitelisted block drivers */
 static int use_bdrv_whitelist;
@@ -1226,9 +1226,10 @@ static int bdrv_child_cb_inactivate(BdrvChild *child)
     return 0;
 }
 
-static bool bdrv_child_cb_change_aio_ctx(BdrvChild *child, AioContext *ctx,
-                                         GHashTable *visited, Transaction *tran,
-                                         Error **errp)
+static bool GRAPH_RDLOCK
+bdrv_child_cb_change_aio_ctx(BdrvChild *child, AioContext *ctx,
+                             GHashTable *visited, Transaction *tran,
+                             Error **errp)
 {
     BlockDriverState *bs = child->opaque;
     return bdrv_change_aio_context(bs, ctx, visited, tran, errp);
@@ -1720,12 +1721,14 @@ bdrv_open_driver(BlockDriverState *bs, BlockDriver *drv, const char *node_name,
 open_failed:
     bs->drv = NULL;
 
+    bdrv_drain_all_begin();
     bdrv_graph_wrlock();
     if (bs->file != NULL) {
         bdrv_unref_child(bs, bs->file);
         assert(!bs->file);
     }
     bdrv_graph_wrunlock();
+    bdrv_drain_all_end();
 
     g_free(bs->opaque);
     bs->opaque = NULL;
@@ -3027,7 +3030,8 @@ static void GRAPH_WRLOCK bdrv_attach_child_common_abort(void *opaque)
     bdrv_replace_child_noperm(s->child, NULL);
 
     if (bdrv_get_aio_context(bs) != s->old_child_ctx) {
-        bdrv_try_change_aio_context(bs, s->old_child_ctx, NULL, &error_abort);
+        bdrv_try_change_aio_context_locked(bs, s->old_child_ctx, NULL,
+                                           &error_abort);
     }
 
     if (bdrv_child_get_parent_aio_context(s->child) != s->old_parent_ctx) {
@@ -3069,6 +3073,9 @@ static TransactionActionDrv bdrv_attach_child_common_drv = {
  *
  * Both @parent_bs and @child_bs can move to a different AioContext in this
  * function.
+ *
+ * All block nodes must be drained before this function is called until after
+ * the transaction is finalized.
  */
 static BdrvChild * GRAPH_WRLOCK
 bdrv_attach_child_common(BlockDriverState *child_bs,
@@ -3112,8 +3119,8 @@ bdrv_attach_child_common(BlockDriverState *child_bs,
     parent_ctx = bdrv_child_get_parent_aio_context(new_child);
     if (child_ctx != parent_ctx) {
         Error *local_err = NULL;
-        int ret = bdrv_try_change_aio_context(child_bs, parent_ctx, NULL,
-                                              &local_err);
+        int ret = bdrv_try_change_aio_context_locked(child_bs, parent_ctx, NULL,
+                                                     &local_err);
 
         if (ret < 0 && child_class->change_aio_ctx) {
             Transaction *aio_ctx_tran = tran_new();
@@ -3179,6 +3186,9 @@ bdrv_attach_child_common(BlockDriverState *child_bs,
  *
  * After calling this function, the transaction @tran may only be completed
  * while holding a writer lock for the graph.
+ *
+ * All block nodes must be drained before this function is called until after
+ * the transaction is finalized.
  */
 static BdrvChild * GRAPH_WRLOCK
 bdrv_attach_child_noperm(BlockDriverState *parent_bs,
@@ -3220,6 +3230,8 @@ bdrv_attach_child_noperm(BlockDriverState *parent_bs,
  *
  * On failure NULL is returned, errp is set and the reference to
  * child_bs is also dropped.
+ *
+ * All block nodes must be drained.
  */
 BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs,
                                   const char *child_name,
@@ -3259,6 +3271,8 @@ out:
  *
  * On failure NULL is returned, errp is set and the reference to
  * child_bs is also dropped.
+ *
+ * All block nodes must be drained.
  */
 BdrvChild *bdrv_attach_child(BlockDriverState *parent_bs,
                              BlockDriverState *child_bs,
@@ -3293,7 +3307,11 @@ out:
     return ret < 0 ? NULL : child;
 }
 
-/* Callers must ensure that child->frozen is false. */
+/*
+ * Callers must ensure that child->frozen is false.
+ *
+ * All block nodes must be drained.
+ */
 void bdrv_root_unref_child(BdrvChild *child)
 {
     BlockDriverState *child_bs = child->bs;
@@ -3314,8 +3332,8 @@ void bdrv_root_unref_child(BdrvChild *child)
          * When the parent requiring a non-default AioContext is removed, the
          * node moves back to the main AioContext
          */
-        bdrv_try_change_aio_context(child_bs, qemu_get_aio_context(), NULL,
-                                    NULL);
+        bdrv_try_change_aio_context_locked(child_bs, qemu_get_aio_context(),
+                                           NULL, NULL);
     }
 
     bdrv_schedule_unref(child_bs);
@@ -3388,7 +3406,11 @@ bdrv_unset_inherits_from(BlockDriverState *root, BdrvChild *child,
     }
 }
 
-/* Callers must ensure that child->frozen is false. */
+/*
+ * Callers must ensure that child->frozen is false.
+ *
+ * All block nodes must be drained.
+ */
 void bdrv_unref_child(BlockDriverState *parent, BdrvChild *child)
 {
     GLOBAL_STATE_CODE();
@@ -3453,6 +3475,9 @@ static BdrvChildRole bdrv_backing_role(BlockDriverState *bs)
  *
  * After calling this function, the transaction @tran may only be completed
  * while holding a writer lock for the graph.
+ *
+ * All block nodes must be drained before this function is called until after
+ * the transaction is finalized.
  */
 static int GRAPH_WRLOCK
 bdrv_set_file_or_backing_noperm(BlockDriverState *parent_bs,
@@ -3545,8 +3570,7 @@ out:
  * Both @bs and @backing_hd can move to a different AioContext in this
  * function.
  *
- * If a backing child is already present (i.e. we're detaching a node), that
- * child node must be drained.
+ * All block nodes must be drained.
  */
 int bdrv_set_backing_hd_drained(BlockDriverState *bs,
                                 BlockDriverState *backing_hd,
@@ -3575,21 +3599,14 @@ out:
 int bdrv_set_backing_hd(BlockDriverState *bs, BlockDriverState *backing_hd,
                         Error **errp)
 {
-    BlockDriverState *drain_bs;
     int ret;
     GLOBAL_STATE_CODE();
 
-    bdrv_graph_rdlock_main_loop();
-    drain_bs = bs->backing ? bs->backing->bs : bs;
-    bdrv_graph_rdunlock_main_loop();
-
-    bdrv_ref(drain_bs);
-    bdrv_drained_begin(drain_bs);
+    bdrv_drain_all_begin();
     bdrv_graph_wrlock();
     ret = bdrv_set_backing_hd_drained(bs, backing_hd, errp);
     bdrv_graph_wrunlock();
-    bdrv_drained_end(drain_bs);
-    bdrv_unref(drain_bs);
+    bdrv_drain_all_end();
 
     return ret;
 }
@@ -3780,10 +3797,12 @@ static BdrvChild *bdrv_open_child_common(const char *filename,
         return NULL;
     }
 
+    bdrv_drain_all_begin();
     bdrv_graph_wrlock();
     child = bdrv_attach_child(parent, bs, bdref_key, child_class, child_role,
                               errp);
     bdrv_graph_wrunlock();
+    bdrv_drain_all_end();
 
     return child;
 }
@@ -4358,9 +4377,7 @@ bdrv_recurse_has_child(BlockDriverState *bs, BlockDriverState *child)
  * returns a pointer to bs_queue, which is either the newly allocated
  * bs_queue, or the existing bs_queue being used.
  *
- * bs is drained here and undrained by bdrv_reopen_queue_free().
- *
- * To be called with bs->aio_context locked.
+ * bs must be drained.
  */
 static BlockReopenQueue * GRAPH_RDLOCK
 bdrv_reopen_queue_child(BlockReopenQueue *bs_queue, BlockDriverState *bs,
@@ -4379,12 +4396,7 @@ bdrv_reopen_queue_child(BlockReopenQueue *bs_queue, BlockDriverState *bs,
 
     GLOBAL_STATE_CODE();
 
-    /*
-     * Strictly speaking, draining is illegal under GRAPH_RDLOCK. We know that
-     * we've been called with bdrv_graph_rdlock_main_loop(), though, so it's ok
-     * in practice.
-     */
-    bdrv_drained_begin(bs);
+    assert(bs->quiesce_counter > 0);
 
     if (bs_queue == NULL) {
         bs_queue = g_new0(BlockReopenQueue, 1);
@@ -4519,12 +4531,17 @@ bdrv_reopen_queue_child(BlockReopenQueue *bs_queue, BlockDriverState *bs,
     return bs_queue;
 }
 
-/* To be called with bs->aio_context locked */
 BlockReopenQueue *bdrv_reopen_queue(BlockReopenQueue *bs_queue,
                                     BlockDriverState *bs,
                                     QDict *options, bool keep_old_opts)
 {
     GLOBAL_STATE_CODE();
+
+    if (bs_queue == NULL) {
+        /* Paired with bdrv_drain_all_end() in bdrv_reopen_queue_free(). */
+        bdrv_drain_all_begin();
+    }
+
     GRAPH_RDLOCK_GUARD_MAINLOOP();
 
     return bdrv_reopen_queue_child(bs_queue, bs, options, NULL, 0, false,
@@ -4537,12 +4554,14 @@ void bdrv_reopen_queue_free(BlockReopenQueue *bs_queue)
     if (bs_queue) {
         BlockReopenQueueEntry *bs_entry, *next;
         QTAILQ_FOREACH_SAFE(bs_entry, bs_queue, entry, next) {
-            bdrv_drained_end(bs_entry->state.bs);
             qobject_unref(bs_entry->state.explicit_options);
             qobject_unref(bs_entry->state.options);
             g_free(bs_entry);
         }
         g_free(bs_queue);
+
+        /* Paired with bdrv_drain_all_begin() in bdrv_reopen_queue(). */
+        bdrv_drain_all_end();
     }
 }
 
@@ -4709,6 +4728,9 @@ int bdrv_reopen_set_read_only(BlockDriverState *bs, bool read_only,
  * Return 0 on success, otherwise return < 0 and set @errp.
  *
  * @reopen_state->bs can move to a different AioContext in this function.
+ *
+ * All block nodes must be drained before this function is called until after
+ * the transaction is finalized.
  */
 static int GRAPH_UNLOCKED
 bdrv_reopen_parse_file_or_backing(BDRVReopenState *reopen_state,
@@ -4802,7 +4824,7 @@ bdrv_reopen_parse_file_or_backing(BDRVReopenState *reopen_state,
 
     if (old_child_bs) {
         bdrv_ref(old_child_bs);
-        bdrv_drained_begin(old_child_bs);
+        assert(old_child_bs->quiesce_counter > 0);
     }
 
     bdrv_graph_rdunlock_main_loop();
@@ -4814,7 +4836,6 @@ bdrv_reopen_parse_file_or_backing(BDRVReopenState *reopen_state,
     bdrv_graph_wrunlock();
 
     if (old_child_bs) {
-        bdrv_drained_end(old_child_bs);
         bdrv_unref(old_child_bs);
     }
 
@@ -4843,6 +4864,9 @@ out_rdlock:
  *
  * After calling this function, the transaction @change_child_tran may only be
  * completed while holding a writer lock for the graph.
+ *
+ * All block nodes must be drained before this function is called until after
+ * the transaction is finalized.
  */
 static int GRAPH_UNLOCKED
 bdrv_reopen_prepare(BDRVReopenState *reopen_state, BlockReopenQueue *queue,
@@ -5156,6 +5180,7 @@ static void bdrv_close(BlockDriverState *bs)
         bs->drv = NULL;
     }
 
+    bdrv_drain_all_begin();
     bdrv_graph_wrlock();
     QLIST_FOREACH_SAFE(child, &bs->children, next, next) {
         bdrv_unref_child(bs, child);
@@ -5164,6 +5189,7 @@ static void bdrv_close(BlockDriverState *bs)
     assert(!bs->backing);
     assert(!bs->file);
     bdrv_graph_wrunlock();
+    bdrv_drain_all_end();
 
     g_free(bs->opaque);
     bs->opaque = NULL;
@@ -5489,9 +5515,7 @@ int bdrv_append(BlockDriverState *bs_new, BlockDriverState *bs_top,
     assert(!bs_new->backing);
     bdrv_graph_rdunlock_main_loop();
 
-    bdrv_drained_begin(bs_top);
-    bdrv_drained_begin(bs_new);
-
+    bdrv_drain_all_begin();
     bdrv_graph_wrlock();
 
     child = bdrv_attach_child_noperm(bs_new, bs_top, "backing",
@@ -5513,9 +5537,7 @@ out:
 
     bdrv_refresh_limits(bs_top, NULL, NULL);
     bdrv_graph_wrunlock();
-
-    bdrv_drained_end(bs_top);
-    bdrv_drained_end(bs_new);
+    bdrv_drain_all_end();
 
     return ret;
 }
@@ -6989,6 +7011,8 @@ bdrv_inactivate_recurse(BlockDriverState *bs, bool top_level)
 
     GLOBAL_STATE_CODE();
 
+    assert(bs->quiesce_counter > 0);
+
     if (!bs->drv) {
         return -ENOMEDIUM;
     }
@@ -7032,9 +7056,7 @@ bdrv_inactivate_recurse(BlockDriverState *bs, bool top_level)
         return -EPERM;
     }
 
-    bdrv_drained_begin(bs);
     bs->open_flags |= BDRV_O_INACTIVE;
-    bdrv_drained_end(bs);
 
     /*
      * Update permissions, they may differ for inactive nodes.
@@ -7059,20 +7081,26 @@ int bdrv_inactivate(BlockDriverState *bs, Error **errp)
     int ret;
 
     GLOBAL_STATE_CODE();
-    GRAPH_RDLOCK_GUARD_MAINLOOP();
+
+    bdrv_drain_all_begin();
+    bdrv_graph_rdlock_main_loop();
 
     if (bdrv_has_bds_parent(bs, true)) {
         error_setg(errp, "Node has active parent node");
-        return -EPERM;
+        ret = -EPERM;
+        goto out;
     }
 
     ret = bdrv_inactivate_recurse(bs, true);
     if (ret < 0) {
         error_setg_errno(errp, -ret, "Failed to inactivate node");
-        return ret;
+        goto out;
     }
 
-    return 0;
+out:
+    bdrv_graph_rdunlock_main_loop();
+    bdrv_drain_all_end();
+    return ret;
 }
 
 int bdrv_inactivate_all(void)
@@ -7082,7 +7110,9 @@ int bdrv_inactivate_all(void)
     int ret = 0;
 
     GLOBAL_STATE_CODE();
-    GRAPH_RDLOCK_GUARD_MAINLOOP();
+
+    bdrv_drain_all_begin();
+    bdrv_graph_rdlock_main_loop();
 
     for (bs = bdrv_first(&it); bs; bs = bdrv_next(&it)) {
         /* Nodes with BDS parents are covered by recursion from the last
@@ -7098,6 +7128,9 @@ int bdrv_inactivate_all(void)
         }
     }
 
+    bdrv_graph_rdunlock_main_loop();
+    bdrv_drain_all_end();
+
     return ret;
 }
 
@@ -7278,10 +7311,6 @@ bool bdrv_op_blocker_is_empty(BlockDriverState *bs)
     return true;
 }
 
-/*
- * Must not be called while holding the lock of an AioContext other than the
- * current one.
- */
 void bdrv_img_create(const char *filename, const char *fmt,
                      const char *base_filename, const char *base_fmt,
                      char *options, uint64_t img_size, int flags, bool quiet,
@@ -7568,10 +7597,21 @@ typedef struct BdrvStateSetAioContext {
     BlockDriverState *bs;
 } BdrvStateSetAioContext;
 
-static bool bdrv_parent_change_aio_context(BdrvChild *c, AioContext *ctx,
-                                           GHashTable *visited,
-                                           Transaction *tran,
-                                           Error **errp)
+/*
+ * Changes the AioContext of @child to @ctx and recursively for the associated
+ * block nodes and all their children and parents. Returns true if the change is
+ * possible and the transaction @tran can be continued. Returns false and sets
+ * @errp if not and the transaction must be aborted.
+ *
+ * @visited will accumulate all visited BdrvChild objects. The caller is
+ * responsible for freeing the list afterwards.
+ *
+ * Must be called with the affected block nodes drained.
+ */
+static bool GRAPH_RDLOCK
+bdrv_parent_change_aio_context(BdrvChild *c, AioContext *ctx,
+                               GHashTable *visited, Transaction *tran,
+                               Error **errp)
 {
     GLOBAL_STATE_CODE();
     if (g_hash_table_contains(visited, c)) {
@@ -7596,6 +7636,17 @@ static bool bdrv_parent_change_aio_context(BdrvChild *c, AioContext *ctx,
     return true;
 }
 
+/*
+ * Changes the AioContext of @c->bs to @ctx and recursively for all its children
+ * and parents. Returns true if the change is possible and the transaction @tran
+ * can be continued. Returns false and sets @errp if not and the transaction
+ * must be aborted.
+ *
+ * @visited will accumulate all visited BdrvChild objects. The caller is
+ * responsible for freeing the list afterwards.
+ *
+ * Must be called with the affected block nodes drained.
+ */
 bool bdrv_child_change_aio_context(BdrvChild *c, AioContext *ctx,
                                    GHashTable *visited, Transaction *tran,
                                    Error **errp)
@@ -7611,10 +7662,6 @@ bool bdrv_child_change_aio_context(BdrvChild *c, AioContext *ctx,
 static void bdrv_set_aio_context_clean(void *opaque)
 {
     BdrvStateSetAioContext *state = (BdrvStateSetAioContext *) opaque;
-    BlockDriverState *bs = (BlockDriverState *) state->bs;
-
-    /* Paired with bdrv_drained_begin in bdrv_change_aio_context() */
-    bdrv_drained_end(bs);
 
     g_free(state);
 }
@@ -7642,10 +7689,12 @@ static TransactionActionDrv set_aio_context = {
  *
  * @visited will accumulate all visited BdrvChild objects. The caller is
  * responsible for freeing the list afterwards.
+ *
+ * @bs must be drained.
  */
-static bool bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
-                                    GHashTable *visited, Transaction *tran,
-                                    Error **errp)
+static bool GRAPH_RDLOCK
+bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
+                        GHashTable *visited, Transaction *tran, Error **errp)
 {
     BdrvChild *c;
     BdrvStateSetAioContext *state;
@@ -7656,21 +7705,17 @@ static bool bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
         return true;
     }
 
-    bdrv_graph_rdlock_main_loop();
     QLIST_FOREACH(c, &bs->parents, next_parent) {
         if (!bdrv_parent_change_aio_context(c, ctx, visited, tran, errp)) {
-            bdrv_graph_rdunlock_main_loop();
             return false;
         }
     }
 
     QLIST_FOREACH(c, &bs->children, next) {
         if (!bdrv_child_change_aio_context(c, ctx, visited, tran, errp)) {
-            bdrv_graph_rdunlock_main_loop();
             return false;
         }
     }
-    bdrv_graph_rdunlock_main_loop();
 
     state = g_new(BdrvStateSetAioContext, 1);
     *state = (BdrvStateSetAioContext) {
@@ -7678,8 +7723,7 @@ static bool bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
         .bs = bs,
     };
 
-    /* Paired with bdrv_drained_end in bdrv_set_aio_context_clean() */
-    bdrv_drained_begin(bs);
+    assert(bs->quiesce_counter > 0);
 
     tran_add(tran, &set_aio_context, state);
 
@@ -7692,9 +7736,13 @@ static bool bdrv_change_aio_context(BlockDriverState *bs, AioContext *ctx,
  *
  * If ignore_child is not NULL, that child (and its subgraph) will not
  * be touched.
+ *
+ * Called with the graph lock held.
+ *
+ * Called while all bs are drained.
  */
-int bdrv_try_change_aio_context(BlockDriverState *bs, AioContext *ctx,
-                                BdrvChild *ignore_child, Error **errp)
+int bdrv_try_change_aio_context_locked(BlockDriverState *bs, AioContext *ctx,
+                                       BdrvChild *ignore_child, Error **errp)
 {
     Transaction *tran;
     GHashTable *visited;
@@ -7703,9 +7751,9 @@ int bdrv_try_change_aio_context(BlockDriverState *bs, AioContext *ctx,
 
     /*
      * Recursion phase: go through all nodes of the graph.
-     * Take care of checking that all nodes support changing AioContext
-     * and drain them, building a linear list of callbacks to run if everything
-     * is successful (the transaction itself).
+     * Take care of checking that all nodes support changing AioContext,
+     * building a linear list of callbacks to run if everything is successful
+     * (the transaction itself).
      */
     tran = tran_new();
     visited = g_hash_table_new(NULL, NULL);
@@ -7732,6 +7780,29 @@ int bdrv_try_change_aio_context(BlockDriverState *bs, AioContext *ctx,
     return 0;
 }
 
+/*
+ * Change bs's and recursively all of its parents' and children's AioContext
+ * to the given new context, returning an error if that isn't possible.
+ *
+ * If ignore_child is not NULL, that child (and its subgraph) will not
+ * be touched.
+ */
+int bdrv_try_change_aio_context(BlockDriverState *bs, AioContext *ctx,
+                                BdrvChild *ignore_child, Error **errp)
+{
+    int ret;
+
+    GLOBAL_STATE_CODE();
+
+    bdrv_drain_all_begin();
+    bdrv_graph_rdlock_main_loop();
+    ret = bdrv_try_change_aio_context_locked(bs, ctx, ignore_child, errp);
+    bdrv_graph_rdunlock_main_loop();
+    bdrv_drain_all_end();
+
+    return ret;
+}
+
 void bdrv_add_aio_context_notifier(BlockDriverState *bs,
         void (*attached_aio_context)(AioContext *new_context, void *opaque),
         void (*detach_aio_context)(void *opaque), void *opaque)
@@ -8159,8 +8230,10 @@ char *bdrv_dirname(BlockDriverState *bs, Error **errp)
 }
 
 /*
- * Hot add/remove a BDS's child. So the user can take a child offline when
- * it is broken and take a new child online
+ * Hot add a BDS's child. Used in combination with bdrv_del_child, so the user
+ * can take a child offline when it is broken and take a new child online.
+ *
+ * All block nodes must be drained.
  */
 void bdrv_add_child(BlockDriverState *parent_bs, BlockDriverState *child_bs,
                     Error **errp)
@@ -8200,6 +8273,12 @@ void bdrv_add_child(BlockDriverState *parent_bs, BlockDriverState *child_bs,
     parent_bs->drv->bdrv_add_child(parent_bs, child_bs, errp);
 }
 
+/*
+ * Hot remove a BDS's child. Used in combination with bdrv_add_child, so the
+ * user can take a child offline when it is broken and take a new child online.
+ *
+ * All block nodes must be drained.
+ */
 void bdrv_del_child(BlockDriverState *parent_bs, BdrvChild *child, Error **errp)
 {
     BdrvChild *tmp;