Fix VM state change handlers running out of order

When a VM state change handler changes VM state, other VM state change
handlers can see the state transitions out of order.

bmdma_map(), scsi_disk_init() and virtio_blk_init() install VM state
change handlers to restart DMA.  These handlers can vm_stop() by
running into a write error on a drive with werror=stop.  This throws
the VM state change handler callback into disarray.  Here's an example
case I observed:

0. The virtual IDE drive goes south.  All future writes return errors.

1. Something encounters a write error, and duly stops the VM with
   vm_stop().

2. vm_stop() calls vm_state_notify(0).

3. vm_state_notify() runs the callbacks in list vm_change_state_head.
   It contains ide_dma_restart_cb() installed by bmdma_map().  It also
   contains audio_vm_change_state_handler() installed by audio_init().

4. audio_vm_change_state_handler() stops audio stuff.

5. User continues VM with monitor command "c".  This runs vm_start().

6. vm_start() calls vm_state_notify(1).

7. vm_state_notify() runs the callbacks in vm_change_state_head.

8. ide_dma_restart_cb() happens to come first.  It does its work, runs
   into a write error, and duly stops the VM with vm_stop().

9. vm_stop() runs vm_state_notify(0).

10. vm_state_notify() runs the callbacks in vm_change_state_head.

11. audio_vm_change_state_handler() stops audio stuff.  Which isn't
   running.

12. vm_stop() finishes, ide_dma_restart_cb() finishes, step 7's
   vm_state_notify() resumes running handlers.

13. audio_vm_change_state_handler() starts audio stuff.  Oopsie.

Fix this by moving the actual write from each VM state change handler
into a new bottom half (suggested by Gleb Natapov).

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>

hw/ide.c

@@ -501,6 +501,7 @@ typedef struct BMDMAState {
     QEMUIOVector qiov;
     int64_t sector_num;
     uint32_t nsector;
+    QEMUBH *bh;
 } BMDMAState;
 
 typedef struct PCIIDEState {
@@ -1109,11 +1110,13 @@ static void ide_sector_write(IDEState *s)
     }
 }
 
-static void ide_dma_restart_cb(void *opaque, int running, int reason)
+static void ide_dma_restart_bh(void *opaque)
 {
     BMDMAState *bm = opaque;
-    if (!running)
-        return;
+
+    qemu_bh_delete(bm->bh);
+    bm->bh = NULL;
+
     if (bm->status & BM_STATUS_DMA_RETRY) {
         bm->status &= ~BM_STATUS_DMA_RETRY;
         ide_dma_restart(bm->ide_if);
@@ -1123,6 +1126,19 @@ static void ide_dma_restart_cb(void *opaque, int running, int reason)
     }
 }
 
+static void ide_dma_restart_cb(void *opaque, int running, int reason)
+{
+    BMDMAState *bm = opaque;
+
+    if (!running)
+        return;
+
+    if (!bm->bh) {
+        bm->bh = qemu_bh_new(ide_dma_restart_bh, bm);
+        qemu_bh_schedule(bm->bh);
+    }
+}
+
 static void ide_write_dma_cb(void *opaque, int ret)
 {
     BMDMAState *bm = opaque;

hw/scsi-disk.c

@@ -74,6 +74,7 @@ struct SCSIDeviceState
     scsi_completionfn completion;
     void *opaque;
     char drive_serial_str[21];
+    QEMUBH *bh;
 };
 
 /* Global pool of SCSIRequest structures.  */
@@ -308,12 +309,13 @@ static int scsi_write_data(SCSIDevice *d, uint32_t tag)
     return 0;
 }
 
-static void scsi_dma_restart_cb(void *opaque, int running, int reason)
+static void scsi_dma_restart_bh(void *opaque)
 {
     SCSIDeviceState *s = opaque;
     SCSIRequest *r = s->requests;
-    if (!running)
-        return;
+
+    qemu_bh_delete(s->bh);
+    s->bh = NULL;
 
     while (r) {
         if (r->status & SCSI_REQ_STATUS_RETRY) {
@@ -324,6 +326,19 @@ static void scsi_dma_restart_cb(void *opaque, int running, int reason)
     }
 }
 
+static void scsi_dma_restart_cb(void *opaque, int running, int reason)
+{
+    SCSIDeviceState *s = opaque;
+
+    if (!running)
+        return;
+
+    if (!s->bh) {
+        s->bh = qemu_bh_new(scsi_dma_restart_bh, s);
+        qemu_bh_schedule(s->bh);
+    }
+}
+
 /* Return a pointer to the data buffer.  */
 static uint8_t *scsi_get_buf(SCSIDevice *d, uint32_t tag)
 {

hw/virtio-blk.c

@@ -26,6 +26,7 @@ typedef struct VirtIOBlock
     VirtQueue *vq;
     void *rq;
     char serial_str[BLOCK_SERIAL_STRLEN + 1];
+    QEMUBH *bh;
 } VirtIOBlock;
 
 static VirtIOBlock *to_virtio_blk(VirtIODevice *vdev)
@@ -302,13 +303,13 @@ static void virtio_blk_handle_output(VirtIODevice *vdev, VirtQueue *vq)
      */
 }
 
-static void virtio_blk_dma_restart_cb(void *opaque, int running, int reason)
+static void virtio_blk_dma_restart_bh(void *opaque)
 {
     VirtIOBlock *s = opaque;
     VirtIOBlockReq *req = s->rq;
 
-    if (!running)
-        return;
+    qemu_bh_delete(s->bh);
+    s->bh = NULL;
 
     s->rq = NULL;
 
@@ -318,6 +319,19 @@ static void virtio_blk_dma_restart_cb(void *opaque, int running, int reason)
     }
 }
 
+static void virtio_blk_dma_restart_cb(void *opaque, int running, int reason)
+{
+    VirtIOBlock *s = opaque;
+
+    if (!running)
+        return;
+
+    if (!s->bh) {
+        s->bh = qemu_bh_new(virtio_blk_dma_restart_bh, s);
+        qemu_bh_schedule(s->bh);
+    }
+}
+
 static void virtio_blk_reset(VirtIODevice *vdev)
 {
     /*