[PATCH v3] migration: Don't allow migration if vm is in POSTMIGRATE

[Tuguoyi]

The following steps will cause qemu assertion failure:
- pause vm by executing 'virsh suspend'
- create external snapshot of memory and disk using 'virsh snapshot-create-as'
- doing the above operation again will cause qemu crash

The backtrace looks like:
#0  0x00007fbf958c5c37 in raise () from /lib/x86_64-linux-gnu/libc.so.6
#1  0x00007fbf958c9028 in abort () from /lib/x86_64-linux-gnu/libc.so.6
#2  0x00007fbf958bebf6 in ?? () from /lib/x86_64-linux-gnu/libc.so.6
#3  0x00007fbf958beca2 in __assert_fail () from /lib/x86_64-linux-gnu/libc.so.6
#4  0x000055ca8decd39d in bdrv_inactivate_recurse (bs=0x55ca90c80400) at 
/build/qemu-5.0/block.c:5724
#5  0x000055ca8dece967 in bdrv_inactivate_all () at 
/build//qemu-5.0/block.c:5792
#6  0x000055ca8de5539d in qemu_savevm_state_complete_precopy_non_iterable 
(inactivate_disks=true, in_postcopy=false, f=0x55ca907044b0)
    at /build/qemu-5.0/migration/savevm.c:1401
#7  qemu_savevm_state_complete_precopy (f=0x55ca907044b0, 
iterable_only=iterable_only@entry=false, 
inactivate_disks=inactivate_disks@entry=true)
    at /build/qemu-5.0/migration/savevm.c:1453
#8  0x000055ca8de4f581 in migration_completion (s=0x55ca8f64d9f0) at 
/build/qemu-5.0/migration/migration.c:2941
#9  migration_iteration_run (s=0x55ca8f64d9f0) at 
/build/qemu-5.0/migration/migration.c:3295
#10 migration_thread (opaque=opaque@entry=0x55ca8f64d9f0) at 
/build/qemu-5.0/migration/migration.c:3459
#11 0x000055ca8dfc6716 in qemu_thread_start (args=<optimized out>) at 
/build/qemu-5.0/util/qemu-thread-posix.c:519
#12 0x00007fbf95c5f184 in start_thread () from 
/lib/x86_64-linux-gnu/libpthread.so.0
#13 0x00007fbf9598cbed in clone () from /lib/x86_64-linux-gnu/libc.so.6

When the first migration completes, bs->open_flags will set BDRV_O_INACTIVE
flag by bdrv_inactivate_all(), and during the second migration the
bdrv_inactivate_recurse assert that the bs->open_flags is already
BDRV_O_INACTIVE enabled which cause crash.

As Vladimir suggested, this patch makes migrate_prepare check the state of vm 
and
return error if it is in RUN_STATE_POSTMIGRATE state.

Signed-off-by: Tuguoyi <tu.guoyi@h3c.com>
---
 migration/migration.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/migration/migration.c b/migration/migration.c
index 87a9b59..5e33962 100644
--- a/migration/migration.c
+++ b/migration/migration.c
@@ -2115,6 +2115,12 @@ static bool migrate_prepare(MigrationState *s, bool blk, 
bool blk_inc,
         return false;
     }
 
+    if (runstate_check(RUN_STATE_POSTMIGRATE)) {
+        error_setg(errp, "Can't migrate the vm that was paused due to "
+                   "previous migration");
+        return false;
+    }
+
     if (migration_is_blocked(errp)) {
         return false;
     }
-- 
2.7.4

[Patch v2]: https://lists.gnu.org/archive/html/qemu-devel/2020-12/msg01318.html
[Patch v1]: https://lists.gnu.org/archive/html/qemu-devel/2020-11/msg05950.html