block: Adjust AioContexts when attaching nodes

So far, we only made sure that updating the AioContext of a node
affected the whole subtree. However, if a node is newly attached to a
new parent, we also need to make sure that both the subtree of the node
and the parent are in the same AioContext. This tries to move the new
child node to the parent AioContext and returns an error if this isn't
possible.

BlockBackends now actually apply their AioContext to their root node.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>

block.c

@@ -2249,14 +2249,19 @@ static void bdrv_replace_child(BdrvChild *child, BlockDriverState *new_bs)
  *
  * On failure NULL is returned, errp is set and the reference to
  * child_bs is also dropped.
+ *
+ * The caller must hold the AioContext lock @child_bs, but not that of @ctx
+ * (unless @child_bs is already in @ctx).
  */
 BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs,
                                   const char *child_name,
                                   const BdrvChildRole *child_role,
+                                  AioContext *ctx,
                                   uint64_t perm, uint64_t shared_perm,
                                   void *opaque, Error **errp)
 {
     BdrvChild *child;
+    Error *local_err = NULL;
     int ret;
 
     ret = bdrv_check_update_perm(child_bs, NULL, perm, shared_perm, NULL, errp);
@@ -2276,6 +2281,31 @@ BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs,
         .opaque         = opaque,
     };
 
+    /* If the AioContexts don't match, first try to move the subtree of
+     * child_bs into the AioContext of the new parent. If this doesn't work,
+     * try moving the parent into the AioContext of child_bs instead. */
+    if (bdrv_get_aio_context(child_bs) != ctx) {
+        ret = bdrv_try_set_aio_context(child_bs, ctx, &local_err);
+        if (ret < 0 && child_role->can_set_aio_ctx) {
+            GSList *ignore = g_slist_prepend(NULL, child);;
+            ctx = bdrv_get_aio_context(child_bs);
+            if (child_role->can_set_aio_ctx(child, ctx, &ignore, NULL)) {
+                error_free(local_err);
+                ret = 0;
+                g_slist_free(ignore);
+                ignore = g_slist_prepend(NULL, child);;
+                child_role->set_aio_ctx(child, ctx, &ignore);
+            }
+            g_slist_free(ignore);
+        }
+        if (ret < 0) {
+            error_propagate(errp, local_err);
+            g_free(child);
+            bdrv_abort_perm_update(child_bs);
+            return NULL;
+        }
+    }
+
     /* This performs the matching bdrv_set_perm() for the above check. */
     bdrv_replace_child(child, child_bs);
 
@@ -2289,6 +2319,9 @@ BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs,
  *
  * On failure NULL is returned, errp is set and the reference to
  * child_bs is also dropped.
+ *
+ * If @parent_bs and @child_bs are in different AioContexts, the caller must
+ * hold the AioContext lock for @child_bs, but not for @parent_bs.
  */
 BdrvChild *bdrv_attach_child(BlockDriverState *parent_bs,
                              BlockDriverState *child_bs,
@@ -2302,11 +2335,11 @@ BdrvChild *bdrv_attach_child(BlockDriverState *parent_bs,
     bdrv_get_cumulative_perm(parent_bs, &perm, &shared_perm);
 
     assert(parent_bs->drv);
-    assert(bdrv_get_aio_context(parent_bs) == bdrv_get_aio_context(child_bs));
     bdrv_child_perm(parent_bs, child_bs, NULL, child_role, NULL,
                     perm, shared_perm, &perm, &shared_perm);
 
     child = bdrv_root_attach_child(child_bs, child_name, child_role,
+                                   bdrv_get_aio_context(parent_bs),
                                    perm, shared_perm, parent_bs, errp);
     if (child == NULL) {
         return NULL;

block/block-backend.c

@@ -392,7 +392,7 @@ BlockBackend *blk_new_open(const char *filename, const char *reference,
         return NULL;
     }
 
-    blk->root = bdrv_root_attach_child(bs, "root", &child_root,
+    blk->root = bdrv_root_attach_child(bs, "root", &child_root, blk->ctx,
                                        perm, BLK_PERM_ALL, blk, errp);
     if (!blk->root) {
         blk_unref(blk);
@@ -803,7 +803,7 @@ int blk_insert_bs(BlockBackend *blk, BlockDriverState *bs, Error **errp)
 {
     ThrottleGroupMember *tgm = &blk->public.throttle_group_member;
     bdrv_ref(bs);
-    blk->root = bdrv_root_attach_child(bs, "root", &child_root,
+    blk->root = bdrv_root_attach_child(bs, "root", &child_root, blk->ctx,
                                        blk->perm, blk->shared_perm, blk, errp);
     if (blk->root == NULL) {
         return -EPERM;
@@ -1861,10 +1861,9 @@ AioContext *blk_get_aio_context(BlockBackend *blk)
 {
     BlockDriverState *bs = blk_bs(blk);
 
-    /* FIXME The AioContext of bs and blk can be inconsistent. For the moment,
-     * we prefer the one of bs for compatibility. */
     if (bs) {
-        return bdrv_get_aio_context(blk_bs(blk));
+        AioContext *ctx = bdrv_get_aio_context(blk_bs(blk));
+        assert(ctx == blk->ctx);
     }
 
     return blk->ctx;

blockjob.c

@@ -205,8 +205,14 @@ int block_job_add_bdrv(BlockJob *job, const char *name, BlockDriverState *bs,
     BdrvChild *c;
 
     bdrv_ref(bs);
-    c = bdrv_root_attach_child(bs, name, &child_job, perm, shared_perm,
+    if (job->job.aio_context != qemu_get_aio_context()) {
-                               job, errp);
+        aio_context_release(job->job.aio_context);
+    }
+    c = bdrv_root_attach_child(bs, name, &child_job, job->job.aio_context,
+                               perm, shared_perm, job, errp);
+    if (job->job.aio_context != qemu_get_aio_context()) {
+        aio_context_acquire(job->job.aio_context);
+    }
     if (c == NULL) {
         return -EPERM;
     }

include/block/block_int.h

@@ -1163,6 +1163,7 @@ void hmp_drive_add_node(Monitor *mon, const char *optstr);
 BdrvChild *bdrv_root_attach_child(BlockDriverState *child_bs,
                                   const char *child_name,
                                   const BdrvChildRole *child_role,
+                                  AioContext *ctx,
                                   uint64_t perm, uint64_t shared_perm,
                                   void *opaque, Error **errp);
 void bdrv_root_unref_child(BdrvChild *child);

tests/test-bdrv-drain.c

@@ -912,6 +912,7 @@ static void test_blockjob_common_drain_node(enum drain_type drain_type,
                                   &error_abort);
     blk_target = blk_new(qemu_get_aio_context(), BLK_PERM_ALL, BLK_PERM_ALL);
     blk_insert_bs(blk_target, target, &error_abort);
+    blk_set_allow_aio_context_change(blk_target, true);
 
     aio_context_acquire(ctx);
     tjob = block_job_create("job0", &test_job_driver, NULL, src,
@@ -972,7 +973,7 @@ static void test_blockjob_common_drain_node(enum drain_type drain_type,
     g_assert_false(job->job.paused);
     g_assert_true(job->job.busy); /* We're in qemu_co_sleep_ns() */
 
-    do_drain_begin(drain_type, target);
+    do_drain_begin_unlocked(drain_type, target);
 
     if (drain_type == BDRV_DRAIN_ALL) {
         /* bdrv_drain_all() drains both src and target */
@@ -983,7 +984,7 @@ static void test_blockjob_common_drain_node(enum drain_type drain_type,
     g_assert_true(job->job.paused);
     g_assert_false(job->job.busy); /* The job is paused */
 
-    do_drain_end(drain_type, target);
+    do_drain_end_unlocked(drain_type, target);
 
     if (use_iothread) {
         /* paused is reset in the I/O thread, wait for it */
@@ -1002,6 +1003,7 @@ static void test_blockjob_common_drain_node(enum drain_type drain_type,
 
     if (use_iothread) {
         blk_set_aio_context(blk_src, qemu_get_aio_context(), &error_abort);
+        blk_set_aio_context(blk_target, qemu_get_aio_context(), &error_abort);
     }
     aio_context_release(ctx);