blk/bdrv_drain_all() only takes effect for a single instant and then
resumes block jobs, guest devices, and other external clients like the
NBD server. This can be handy when performing a synchronous drain
before terminating the program, for example.
Monitor commands usually need to quiesce I/O across an entire code
region so blk/bdrv_drain_all() is not suitable. They must use
bdrv_drain_all_begin/end() to mark the region. This prevents new I/O
requests from slipping in or worse - block jobs completing and modifying
the graph.
I audited other blk/bdrv_drain_all() callers but did not find anything
that needs a similar fix. This patch fixes the savevm/loadvm commands.
Although I haven't encountered a read world issue this makes the code
safer.
Suggested-by: Kevin Wolf <kw...@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefa...@redhat.com>
---
migration/savevm.c | 9 ++++++++-
1 file changed, 8 insertions(+), 1 deletion(-)
diff --git a/migration/savevm.c b/migration/savevm.c
index 3ca319f..f134e48 100644
--- a/migration/savevm.c
+++ b/migration/savevm.c
@@ -2113,6 +2113,8 @@ int save_vmstate(const char *name, Error **errp)
}
vm_stop(RUN_STATE_SAVE_VM);
+ bdrv_drain_all_begin();
+
aio_context_acquire(aio_context);
memset(sn, 0, sizeof(*sn));
@@ -2171,6 +2173,9 @@ int save_vmstate(const char *name, Error **errp)
if (aio_context) {
aio_context_release(aio_context);
}
+
+ bdrv_drain_all_end();
+
if (saved_vm_running) {
vm_start();
}
@@ -2279,7 +2284,7 @@ int load_vmstate(const char *name, Error **errp)
}
/* Flush all IO requests so they don't interfere with the new state. */
- bdrv_drain_all();
+ bdrv_drain_all_begin();
ret = bdrv_all_goto_snapshot(name, &bs);
if (ret < 0) {
@@ -2303,6 +2308,8 @@ int load_vmstate(const char *name, Error **errp)
qemu_fclose(f);
aio_context_release(aio_context);
+ bdrv_drain_all_end();
+
migration_incoming_state_destroy();
if (ret < 0) {
error_setg(errp, "Error %d while loading VM state", ret);
--
2.9.3
