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

From af295ac08ffca4efd6f10a2d1a38eaa8d09d8e6f Mon Sep 17 00:00:00 2001
From: Fiona Ebner <f.ebner@proxmox.com>
Date: Fri, 30 May 2025 17:10:51 +0200
Subject: [PATCH 26/33] block: move drain outside of quorum_add_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: [14/21] 1079c7996e39d09d021d210d974522fba2e5515d (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".

The quorum_add_child() callback runs under the graph lock, so it is
not allowed to drain. It is only called as the .bdrv_add_child()
callback, which is only called in the bdrv_add_child() function, which
also runs under the graph lock.

The bdrv_add_child() function is called by qmp_x_blockdev_change(),
where a drained section is introduced.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Message-ID: <20250530151125.955508-15-f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
(cherry picked from commit 0414930d3adfa89299eaea5ce92accab15d9fba5)
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
---
 block.c                          | 10 ++++++++--
 block/quorum.c                   |  2 --
 blockdev.c                       |  2 ++
 include/block/block_int-common.h |  7 +++++++
 4 files changed, 17 insertions(+), 4 deletions(-)

diff --git a/block.c b/block.c
index 3857f42877..d7425ff971 100644
--- a/block.c
+++ b/block.c
@@ -8220,8 +8220,10 @@ 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)
@@ -8261,6 +8263,10 @@ 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.
+ */
 void bdrv_del_child(BlockDriverState *parent_bs, BdrvChild *child, Error **errp)
 {
     BdrvChild *tmp;
diff --git a/block/quorum.c b/block/quorum.c
index ea17b0ec13..ed8ce801ee 100644
--- a/block/quorum.c
+++ b/block/quorum.c
@@ -1096,10 +1096,8 @@ quorum_add_child(BlockDriverState *bs, BlockDriverState *child_bs, Error **errp)
     /* We can safely add the child now */
     bdrv_ref(child_bs);
 
-    bdrv_drain_all_begin();
     child = bdrv_attach_child(bs, child_bs, indexstr, &child_of_bds,
                               BDRV_CHILD_DATA, errp);
-    bdrv_drain_all_end();
     if (child == NULL) {
         s->next_child_index--;
         return;
diff --git a/blockdev.c b/blockdev.c
index 41b6481c9a..8edd3e7bba 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -3525,6 +3525,7 @@ void qmp_x_blockdev_change(const char *parent, const char *child,
     BlockDriverState *parent_bs, *new_bs = NULL;
     BdrvChild *p_child;
 
+    bdrv_drain_all_begin();
     bdrv_graph_wrlock();
 
     parent_bs = bdrv_lookup_bs(parent, parent, errp);
@@ -3562,6 +3563,7 @@ void qmp_x_blockdev_change(const char *parent, const char *child,
 
 out:
     bdrv_graph_wrunlock();
+    bdrv_drain_all_end();
 }
 
 BlockJobInfoList *qmp_query_block_jobs(Error **errp)
diff --git a/include/block/block_int-common.h b/include/block/block_int-common.h
index 5d34e6a510..8d76b37c03 100644
--- a/include/block/block_int-common.h
+++ b/include/block/block_int-common.h
@@ -396,6 +396,13 @@ struct BlockDriver {
     int GRAPH_RDLOCK_PTR (*bdrv_probe_geometry)(
         BlockDriverState *bs, HDGeometry *geo);
 
+    /**
+     * 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 GRAPH_WRLOCK_PTR (*bdrv_add_child)(
         BlockDriverState *parent, BlockDriverState *child, Error **errp);
 
-- 
2.39.3