block: bdrv_reopen_multiple(): move bdrv_flush to separate pre-prepare

During reopen we may add backing bs from other aio context, which may
lead to changing original context of top bs.

We are going to move graph modification to prepare stage. So, it will
be possible that bdrv_flush() in bdrv_reopen_prepare called on bs in
non-original aio context, which we didn't aquire which leads to crash.

To avoid this problem move bdrv_flush() to be a separate reopen stage
before bdrv_reopen_prepare().

This doesn't seem correct to acquire only one aio context and not all
contexts participating in reopen. But it's not obvious how to do it
correctly, keeping in mind:

 1. rules of bdrv_set_aio_context_ignore() that requires new_context
    lock not being held

 2. possible deadlocks because of holding all (or several?) AioContext
    locks

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20210428151804.439460-30-vsementsov@virtuozzo.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

diff --git a/block.c b/block.c
index bdfe59d94d268c2f3c7d579b2820e6225ac6facc..357ec1be2c3f15e3ff07f9e1fbb0605b6cc7a9e8 100644 (file)
--- a/block.c
+++ b/block.c
@@ -4274,6 +4274,14 @@ int bdrv_reopen_multiple(BlockReopenQueue *bs_queue, Error **errp)
 
     assert(bs_queue != NULL);
 
+    QTAILQ_FOREACH(bs_entry, bs_queue, entry) {
+        ret = bdrv_flush(bs_entry->state.bs);
+        if (ret < 0) {
+            error_setg_errno(errp, -ret, "Error flushing drive");
+            goto cleanup;
+        }
+    }
+
     QTAILQ_FOREACH(bs_entry, bs_queue, entry) {
         assert(bs_entry->state.bs->quiesce_counter > 0);
         if (bdrv_reopen_prepare(&bs_entry->state, bs_queue, errp)) {
@@ -4669,12 +4677,6 @@ static int bdrv_reopen_prepare(BDRVReopenState *reopen_state,
     bdrv_reopen_perm(queue, reopen_state->bs,
                      &reopen_state->perm, &reopen_state->shared_perm);
 
-    ret = bdrv_flush(reopen_state->bs);
-    if (ret) {
-        error_setg_errno(errp, -ret, "Error flushing drive");
-        goto error;
-    }
-
     if (drv->bdrv_reopen_prepare) {
         /*
          * If a driver-specific option is missing, it means that we