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block: move drain outside of bdrv_try_change_aio_context()

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

Convert the function to a _locked() version that has to be called with
the graph lock held and add a convenience wrapper that has to be
called with the graph unlocked, which drains and takes the lock
itself. Since bdrv_try_change_aio_context() is global state code, the
wrapper is too.

Callers are adapted to use the appropriate variant, depending on
whether the caller already holds the lock. In the
test_set_aio_context() unit test, prior drains can be removed, because
draining already happens inside the new wrapper.

Note that bdrv_attach_child_common_abort(), bdrv_attach_child_common()
and bdrv_root_unref_child() hold the graph lock and are not actually
allowed to drain either. This will be addressed in the following
commits.

Functions like qmp_blockdev_mirror() query the nodes to act on before
draining and locking. In theory, draining could invalidate those nodes.
This kind of issue is not addressed by these commits.

Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-ID: <20250530151125.955508-10-f.ebner@proxmox.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
This commit is contained in:
Fiona Ebner 2025-05-30 17:10:46 +02:00 • committed by Kevin Wolf
parent 91ba0e1c38
commit a1ea8eb591
4 changed files with 59 additions and 26 deletions
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15
blockdev.c
View file

@ -3601,12 +3601,13 @@ void qmp_x_blockdev_set_iothread(const char *node_name, StrOrNull *iothread,
AioContext *new_context;
BlockDriverState *bs;
GRAPH_RDLOCK_GUARD_MAINLOOP();
bdrv_drain_all_begin();
bdrv_graph_rdlock_main_loop();
bs = bdrv_find_node(node_name);
if (!bs) {
error_setg(errp, "Failed to find node with node-name='%s'", node_name);
return;
goto out;
}
/* Protects against accidents. */
@ -3614,14 +3615,14 @@ void qmp_x_blockdev_set_iothread(const char *node_name, StrOrNull *iothread,
error_setg(errp, "Node %s is associated with a BlockBackend and could "
"be in use (use force=true to override this check)",
node_name);
return;
goto out;
}
if (iothread->type == QTYPE_QSTRING) {
IOThread *obj = iothread_by_id(iothread->u.s);
if (!obj) {
error_setg(errp, "Cannot find iothread %s", iothread->u.s);
return;
goto out;
}
new_context = iothread_get_aio_context(obj);
@ -3629,7 +3630,11 @@ void qmp_x_blockdev_set_iothread(const char *node_name, StrOrNull *iothread,
new_context = qemu_get_aio_context();
}
bdrv_try_change_aio_context(bs, new_context, NULL, errp);
bdrv_try_change_aio_context_locked(bs, new_context, NULL, errp);
out:
bdrv_graph_rdunlock_main_loop();
bdrv_drain_all_end();
}
QemuOptsList qemu_common_drive_opts = {