[Qemu-devel] [RFC for-2.7] block: keep AioContext pointer in BlockBackend

[Stefan Hajnoczi <stefanha@redhat.com>]

blk_get/set_aio_context() delegate to BlockDriverState without storing
the AioContext pointer in BlockBackend.

There are two flaws:

1. BlockBackend falls back to the QEMU main loop AioContext when there
   is no root BlockDriverState.  This means the drive loses its
   AioContext during media change and would break dataplane.

2. BlockBackend state used from multiple threads has no lock.  Race
   conditions will creep in as functionality is moved from
   BlockDriverState to BlockBackend due to the absense of a lock.  The
   monitor cannot access BlockBackend state safely while an IOThread is
   also accessing the state.

Both issue #1 and #2 are mostly theoretical at the moment.  I haven't
figured out a way to trigger #1 with virtio-blk (does not support media
change) or virtio-scsi (blocks the eject operation).  #2 may be possible
with block accounting statistics in BlockBackend but I'm not aware of a
crash that can be triggered.

This patch stores the AioContext pointer in BlockBackend and puts newly
inserted BlockDriverStates into the AioContext.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
 block/block-backend.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/block/block-backend.c b/block/block-backend.c
index d74f670..e39c583 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -30,6 +30,7 @@ static AioContext *blk_aiocb_get_aio_context(BlockAIOCB *acb);
 struct BlockBackend {
     char *name;
     int refcnt;
+    AioContext *aio_context;
     BdrvChild *root;
     DriveInfo *legacy_dinfo;    /* null unless created by drive_new() */
     QTAILQ_ENTRY(BlockBackend) link;         /* for block_backends */
@@ -106,6 +107,7 @@ BlockBackend *blk_new(Error **errp)
 
     blk = g_new0(BlockBackend, 1);
     blk->refcnt = 1;
+    blk->aio_context = qemu_get_aio_context();
     notifier_list_init(&blk->remove_bs_notifiers);
     notifier_list_init(&blk->insert_bs_notifiers);
     QTAILQ_INSERT_TAIL(&block_backends, blk, link);
@@ -433,6 +435,7 @@ void blk_insert_bs(BlockBackend *blk, BlockDriverState *bs)
 {
     assert(!blk->root && !bs->blk);
     bdrv_ref(bs);
+    bdrv_set_aio_context(bs, blk->aio_context);
     blk->root = bdrv_root_attach_child(bs, "root", &child_root);
     bs->blk = blk;
 
@@ -1348,13 +1351,7 @@ void blk_op_unblock_all(BlockBackend *blk, Error *reason)
 
 AioContext *blk_get_aio_context(BlockBackend *blk)
 {
-    BlockDriverState *bs = blk_bs(blk);
-
-    if (bs) {
-        return bdrv_get_aio_context(bs);
-    } else {
-        return qemu_get_aio_context();
-    }
+    return blk->aio_context;
 }
 
 static AioContext *blk_aiocb_get_aio_context(BlockAIOCB *acb)
@@ -1367,6 +1364,8 @@ void blk_set_aio_context(BlockBackend *blk, AioContext *new_context)
 {
     BlockDriverState *bs = blk_bs(blk);
 
+    blk->aio_context = new_context;
+
     if (bs) {
         bdrv_set_aio_context(bs, new_context);
     }
-- 
2.5.5