qemu-kvm/kvm-block-move-drain-outside-of-bdrv_root_unref_child.patch

From 1ad00825750a515baf5bd9185bbc31549a61568b Mon Sep 17 00:00:00 2001
From: Fiona Ebner <f.ebner@proxmox.com>
Date: Fri, 30 May 2025 17:10:52 +0200
Subject: [PATCH 27/33] block: move drain outside of bdrv_root_unref_child()

RH-Author: Kevin Wolf <kwolf@redhat.com>
RH-MergeRequest: 393: block: do not drain while holding the graph lock
RH-Jira: RHEL-88561
RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>
RH-Acked-by: Hanna Czenczek <hreitz@redhat.com>
RH-Commit: [15/21] 05cc374ad5ce3f3ae4b3e5d4024c67141acf2cc2 (kmwolf/centos-qemu-kvm)

This is part of resolving the deadlock mentioned in commit "block:
move draining out of bdrv_change_aio_context() and mark GRAPH_RDLOCK".

bdrv_root_unref_child() is called by:
1. blk_remove_bs(), where a drained section is introduced.
2. bdrv_unref_child(), which runs under the graph lock, so the drain
   will be moved further up to its callers.
3. block_job_remove_all_bdrv(), where a drained section is introduced.

For all callers of bdrv_unref_child() and its generated
bdrv_co_unref_child() coroutine variant, a drained section is
introduced, they are not explicilty listed here. The caller
quorum_del_child() holds the graph lock, so it is not actually allowed
to drain. This will be addressed in the next commit.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20250530151125.955508-16-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit b13f54654546cbc0661d3fe9d25f7543535c2bee)
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block.c                      | 18 ++++++++++++++----
 block/blklogwrites.c         |  4 ++++
 block/blkverify.c            |  2 ++
 block/block-backend.c        |  2 ++
 block/qcow2.c                |  4 ++++
 block/quorum.c               |  6 ++++++
 block/replication.c          |  2 ++
 block/snapshot.c             |  2 ++
 block/vmdk.c                 | 10 ++++++++++
 blockjob.c                   |  2 ++
 tests/unit/test-bdrv-drain.c |  4 ++++
 11 files changed, 52 insertions(+), 4 deletions(-)

diff --git a/block.c b/block.c
index d7425ff971..51bc084b1e 100644
--- a/block.c
+++ b/block.c
@@ -1721,12 +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;
@@ -3305,7 +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;
@@ -3326,10 +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_drain_all_begin();
         bdrv_try_change_aio_context_locked(child_bs, qemu_get_aio_context(),
                                            NULL, NULL);
-        bdrv_drain_all_end();
     }
 
     bdrv_schedule_unref(child_bs);
@@ -3402,7 +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();
@@ -5172,6 +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);
@@ -5180,6 +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;
diff --git a/block/blklogwrites.c b/block/blklogwrites.c
index b0f78c4bc7..70ac76f401 100644
--- a/block/blklogwrites.c
+++ b/block/blklogwrites.c
@@ -281,9 +281,11 @@ static int blk_log_writes_open(BlockDriverState *bs, QDict *options, int flags,
     ret = 0;
 fail_log:
     if (ret < 0) {
+        bdrv_drain_all_begin();
         bdrv_graph_wrlock();
         bdrv_unref_child(bs, s->log_file);
         bdrv_graph_wrunlock();
+        bdrv_drain_all_end();
         s->log_file = NULL;
         qemu_mutex_destroy(&s->mutex);
     }
@@ -296,10 +298,12 @@ static void blk_log_writes_close(BlockDriverState *bs)
 {
     BDRVBlkLogWritesState *s = bs->opaque;
 
+    bdrv_drain_all_begin();
     bdrv_graph_wrlock();
     bdrv_unref_child(bs, s->log_file);
     s->log_file = NULL;
     bdrv_graph_wrunlock();
+    bdrv_drain_all_end();
     qemu_mutex_destroy(&s->mutex);
 }
 
diff --git a/block/blkverify.c b/block/blkverify.c
index db79a36681..3a71f7498c 100644
--- a/block/blkverify.c
+++ b/block/blkverify.c
@@ -151,10 +151,12 @@ static void blkverify_close(BlockDriverState *bs)
 {
     BDRVBlkverifyState *s = bs->opaque;
 
+    bdrv_drain_all_begin();
     bdrv_graph_wrlock();
     bdrv_unref_child(bs, s->test_file);
     s->test_file = NULL;
     bdrv_graph_wrunlock();
+    bdrv_drain_all_end();
 }
 
 static int64_t coroutine_fn GRAPH_RDLOCK
diff --git a/block/block-backend.c b/block/block-backend.c
index 24cae3cb55..68209bb2f7 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -889,9 +889,11 @@ void blk_remove_bs(BlockBackend *blk)
     root = blk->root;
     blk->root = NULL;
 
+    bdrv_drain_all_begin();
     bdrv_graph_wrlock();
     bdrv_root_unref_child(root);
     bdrv_graph_wrunlock();
+    bdrv_drain_all_end();
 }
 
 /*
diff --git a/block/qcow2.c b/block/qcow2.c
index 9fc96ba99a..9480598b6d 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1901,7 +1901,9 @@ qcow2_do_open(BlockDriverState *bs, QDict *options, int flags,
     g_free(s->image_data_file);
     if (open_data_file && has_data_file(bs)) {
         bdrv_graph_co_rdunlock();
+        bdrv_drain_all_begin();
         bdrv_co_unref_child(bs, s->data_file);
+        bdrv_drain_all_end();
         bdrv_graph_co_rdlock();
         s->data_file = NULL;
     }
@@ -2827,9 +2829,11 @@ qcow2_do_close(BlockDriverState *bs, bool close_data_file)
     if (close_data_file && has_data_file(bs)) {
         GLOBAL_STATE_CODE();
         bdrv_graph_rdunlock_main_loop();
+        bdrv_drain_all_begin();
         bdrv_graph_wrlock();
         bdrv_unref_child(bs, s->data_file);
         bdrv_graph_wrunlock();
+        bdrv_drain_all_end();
         s->data_file = NULL;
         bdrv_graph_rdlock_main_loop();
     }
diff --git a/block/quorum.c b/block/quorum.c
index ed8ce801ee..81407a38ee 100644
--- a/block/quorum.c
+++ b/block/quorum.c
@@ -1037,6 +1037,7 @@ static int quorum_open(BlockDriverState *bs, QDict *options, int flags,
 
 close_exit:
     /* cleanup on error */
+    bdrv_drain_all_begin();
     bdrv_graph_wrlock();
     for (i = 0; i < s->num_children; i++) {
         if (!opened[i]) {
@@ -1045,6 +1046,7 @@ close_exit:
         bdrv_unref_child(bs, s->children[i]);
     }
     bdrv_graph_wrunlock();
+    bdrv_drain_all_end();
     g_free(s->children);
     g_free(opened);
 exit:
@@ -1057,11 +1059,13 @@ static void quorum_close(BlockDriverState *bs)
     BDRVQuorumState *s = bs->opaque;
     int i;
 
+    bdrv_drain_all_begin();
     bdrv_graph_wrlock();
     for (i = 0; i < s->num_children; i++) {
         bdrv_unref_child(bs, s->children[i]);
     }
     bdrv_graph_wrunlock();
+    bdrv_drain_all_end();
 
     g_free(s->children);
 }
@@ -1143,7 +1147,9 @@ quorum_del_child(BlockDriverState *bs, BdrvChild *child, Error **errp)
             (s->num_children - i - 1) * sizeof(BdrvChild *));
     s->children = g_renew(BdrvChild *, s->children, --s->num_children);
 
+    bdrv_drain_all_begin();
     bdrv_unref_child(bs, child);
+    bdrv_drain_all_end();
 
     quorum_refresh_flags(bs);
 }
diff --git a/block/replication.c b/block/replication.c
index 02814578c6..92eb432b1b 100644
--- a/block/replication.c
+++ b/block/replication.c
@@ -655,12 +655,14 @@ static void replication_done(void *opaque, int ret)
     if (ret == 0) {
         s->stage = BLOCK_REPLICATION_DONE;
 
+        bdrv_drain_all_begin();
         bdrv_graph_wrlock();
         bdrv_unref_child(bs, s->secondary_disk);
         s->secondary_disk = NULL;
         bdrv_unref_child(bs, s->hidden_disk);
         s->hidden_disk = NULL;
         bdrv_graph_wrunlock();
+        bdrv_drain_all_end();
 
         s->error = 0;
     } else {
diff --git a/block/snapshot.c b/block/snapshot.c
index 9f300a78bd..28c9c43621 100644
--- a/block/snapshot.c
+++ b/block/snapshot.c
@@ -291,9 +291,11 @@ int bdrv_snapshot_goto(BlockDriverState *bs,
         }
 
         /* .bdrv_open() will re-attach it */
+        bdrv_drain_all_begin();
         bdrv_graph_wrlock();
         bdrv_unref_child(bs, fallback);
         bdrv_graph_wrunlock();
+        bdrv_drain_all_end();
 
         ret = bdrv_snapshot_goto(fallback_bs, snapshot_id, errp);
         memset(bs->opaque, 0, drv->instance_size);
diff --git a/block/vmdk.c b/block/vmdk.c
index 9c7ab037e1..89a7250120 100644
--- a/block/vmdk.c
+++ b/block/vmdk.c
@@ -271,6 +271,7 @@ static void vmdk_free_extents(BlockDriverState *bs)
     BDRVVmdkState *s = bs->opaque;
     VmdkExtent *e;
 
+    bdrv_drain_all_begin();
     bdrv_graph_wrlock();
     for (i = 0; i < s->num_extents; i++) {
         e = &s->extents[i];
@@ -283,6 +284,7 @@ static void vmdk_free_extents(BlockDriverState *bs)
         }
     }
     bdrv_graph_wrunlock();
+    bdrv_drain_all_end();
 
     g_free(s->extents);
 }
@@ -1247,9 +1249,11 @@ vmdk_parse_extents(const char *desc, BlockDriverState *bs, QDict *options,
                             0, 0, 0, 0, 0, &extent, errp);
             if (ret < 0) {
                 bdrv_graph_rdunlock_main_loop();
+                bdrv_drain_all_begin();
                 bdrv_graph_wrlock();
                 bdrv_unref_child(bs, extent_file);
                 bdrv_graph_wrunlock();
+                bdrv_drain_all_end();
                 bdrv_graph_rdlock_main_loop();
                 goto out;
             }
@@ -1266,9 +1270,11 @@ vmdk_parse_extents(const char *desc, BlockDriverState *bs, QDict *options,
             g_free(buf);
             if (ret) {
                 bdrv_graph_rdunlock_main_loop();
+                bdrv_drain_all_begin();
                 bdrv_graph_wrlock();
                 bdrv_unref_child(bs, extent_file);
                 bdrv_graph_wrunlock();
+                bdrv_drain_all_end();
                 bdrv_graph_rdlock_main_loop();
                 goto out;
             }
@@ -1277,9 +1283,11 @@ vmdk_parse_extents(const char *desc, BlockDriverState *bs, QDict *options,
             ret = vmdk_open_se_sparse(bs, extent_file, bs->open_flags, errp);
             if (ret) {
                 bdrv_graph_rdunlock_main_loop();
+                bdrv_drain_all_begin();
                 bdrv_graph_wrlock();
                 bdrv_unref_child(bs, extent_file);
                 bdrv_graph_wrunlock();
+                bdrv_drain_all_end();
                 bdrv_graph_rdlock_main_loop();
                 goto out;
             }
@@ -1287,9 +1295,11 @@ vmdk_parse_extents(const char *desc, BlockDriverState *bs, QDict *options,
         } else {
             error_setg(errp, "Unsupported extent type '%s'", type);
             bdrv_graph_rdunlock_main_loop();
+            bdrv_drain_all_begin();
             bdrv_graph_wrlock();
             bdrv_unref_child(bs, extent_file);
             bdrv_graph_wrunlock();
+            bdrv_drain_all_end();
             bdrv_graph_rdlock_main_loop();
             ret = -ENOTSUP;
             goto out;
diff --git a/blockjob.c b/blockjob.c
index 44991e3ff7..e68181a35b 100644
--- a/blockjob.c
+++ b/blockjob.c
@@ -198,6 +198,7 @@ void block_job_remove_all_bdrv(BlockJob *job)
      * one to make sure that such a concurrent access does not attempt
      * to process an already freed BdrvChild.
      */
+    bdrv_drain_all_begin();
     bdrv_graph_wrlock();
     while (job->nodes) {
         GSList *l = job->nodes;
@@ -211,6 +212,7 @@ void block_job_remove_all_bdrv(BlockJob *job)
         g_slist_free_1(l);
     }
     bdrv_graph_wrunlock();
+    bdrv_drain_all_end();
 }
 
 bool block_job_has_bdrv(BlockJob *job, BlockDriverState *bs)
diff --git a/tests/unit/test-bdrv-drain.c b/tests/unit/test-bdrv-drain.c
index ac76525e5a..59c2793725 100644
--- a/tests/unit/test-bdrv-drain.c
+++ b/tests/unit/test-bdrv-drain.c
@@ -955,11 +955,13 @@ static void bdrv_test_top_close(BlockDriverState *bs)
 {
     BdrvChild *c, *next_c;
 
+    bdrv_drain_all_begin();
     bdrv_graph_wrlock();
     QLIST_FOREACH_SAFE(c, &bs->children, next, next_c) {
         bdrv_unref_child(bs, c);
     }
     bdrv_graph_wrunlock();
+    bdrv_drain_all_end();
 }
 
 static int coroutine_fn GRAPH_RDLOCK
@@ -1016,7 +1018,9 @@ static void coroutine_fn test_co_delete_by_drain(void *opaque)
         bdrv_graph_co_rdlock();
         QLIST_FOREACH_SAFE(c, &bs->children, next, next_c) {
             bdrv_graph_co_rdunlock();
+            bdrv_drain_all_begin();
             bdrv_co_unref_child(bs, c);
+            bdrv_drain_all_end();
             bdrv_graph_co_rdlock();
         }
         bdrv_graph_co_rdunlock();
-- 
2.39.3