[Qemu-devel] [PATCH 3/4] blockdev: acquire AioContext in QMP 'transaction' actions

[Stefan Hajnoczi]

The transaction QMP command performs operations atomically on a group of
drives.  This command needs to acquire AioContext in order to work
safely when virtio-blk dataplane IOThreads are accessing drives.

The transactional nature of the command means that actions are split
into prepare, commit, abort, and clean functions.  Acquire the
AioContext in prepare and don't release it until one of the other
functions is called.  This prevents the IOThread from running the
AioContext before the transaction has completed.

Signed-off-by: Stefan Hajnoczi <address@hidden>
---
 blockdev.c                      | 52 ++++++++++++++++++++++++++++++++++++++++-
 hw/block/dataplane/virtio-blk.c |  2 ++
 2 files changed, 53 insertions(+), 1 deletion(-)

diff --git a/blockdev.c b/blockdev.c
index 0d06983..90cb33d 100644
--- a/blockdev.c
+++ b/blockdev.c
@@ -1193,6 +1193,7 @@ struct BlkTransactionState {
 typedef struct InternalSnapshotState {
     BlkTransactionState common;
     BlockDriverState *bs;
+    AioContext *aio_context;
     QEMUSnapshotInfo sn;
 } InternalSnapshotState;
 
@@ -1226,6 +1227,10 @@ static void 
internal_snapshot_prepare(BlkTransactionState *common,
         return;
     }
 
+    /* AioContext is released in .clean() */
+    state->aio_context = bdrv_get_aio_context(bs);
+    aio_context_acquire(state->aio_context);
+
     if (!bdrv_is_inserted(bs)) {
         error_set(errp, QERR_DEVICE_HAS_NO_MEDIUM, device);
         return;
@@ -1303,11 +1308,22 @@ static void internal_snapshot_abort(BlkTransactionState 
*common)
     }
 }
 
+static void internal_snapshot_clean(BlkTransactionState *common)
+{
+    InternalSnapshotState *state = DO_UPCAST(InternalSnapshotState,
+                                             common, common);
+
+    if (state->aio_context) {
+        aio_context_release(state->aio_context);
+    }
+}
+
 /* external snapshot private data */
 typedef struct ExternalSnapshotState {
     BlkTransactionState common;
     BlockDriverState *old_bs;
     BlockDriverState *new_bs;
+    AioContext *aio_context;
 } ExternalSnapshotState;
 
 static void external_snapshot_prepare(BlkTransactionState *common,
@@ -1374,6 +1390,10 @@ static void 
external_snapshot_prepare(BlkTransactionState *common,
         return;
     }
 
+    /* Acquire AioContext now so any threads operating on old_bs stop */
+    state->aio_context = bdrv_get_aio_context(state->old_bs);
+    aio_context_acquire(state->aio_context);
+
     if (!bdrv_is_inserted(state->old_bs)) {
         error_set(errp, QERR_DEVICE_HAS_NO_MEDIUM, device);
         return;
@@ -1432,6 +1452,8 @@ static void external_snapshot_commit(BlkTransactionState 
*common)
     ExternalSnapshotState *state =
                              DO_UPCAST(ExternalSnapshotState, common, common);
 
+    bdrv_set_aio_context(state->new_bs, state->aio_context);
+
     /* This removes our old bs and adds the new bs */
     bdrv_append(state->new_bs, state->old_bs);
     /* We don't need (or want) to use the transactional
@@ -1439,6 +1461,8 @@ static void external_snapshot_commit(BlkTransactionState 
*common)
      * don't want to abort all of them if one of them fails the reopen */
     bdrv_reopen(state->new_bs, state->new_bs->open_flags & ~BDRV_O_RDWR,
                 NULL);
+
+    aio_context_release(state->aio_context);
 }
 
 static void external_snapshot_abort(BlkTransactionState *common)
@@ -1448,23 +1472,38 @@ static void external_snapshot_abort(BlkTransactionState 
*common)
     if (state->new_bs) {
         bdrv_unref(state->new_bs);
     }
+    if (state->aio_context) {
+        aio_context_release(state->aio_context);
+    }
 }
 
 typedef struct DriveBackupState {
     BlkTransactionState common;
     BlockDriverState *bs;
+    AioContext *aio_context;
     BlockJob *job;
 } DriveBackupState;
 
 static void drive_backup_prepare(BlkTransactionState *common, Error **errp)
 {
     DriveBackupState *state = DO_UPCAST(DriveBackupState, common, common);
+    BlockDriverState *bs;
     DriveBackup *backup;
     Error *local_err = NULL;
 
     assert(common->action->kind == TRANSACTION_ACTION_KIND_DRIVE_BACKUP);
     backup = common->action->drive_backup;
 
+    bs = bdrv_find(backup->device);
+    if (!bs) {
+        error_set(errp, QERR_DEVICE_NOT_FOUND, backup->device);
+        return;
+    }
+
+    /* AioContext is released in .clean() */
+    state->aio_context = bdrv_get_aio_context(bs);
+    aio_context_acquire(state->aio_context);
+
     qmp_drive_backup(backup->device, backup->target,
                      backup->has_format, backup->format,
                      backup->sync,
@@ -1478,7 +1517,7 @@ static void drive_backup_prepare(BlkTransactionState 
*common, Error **errp)
         return;
     }
 
-    state->bs = bdrv_find(backup->device);
+    state->bs = bs;
     state->job = state->bs->job;
 }
 
@@ -1493,6 +1532,15 @@ static void drive_backup_abort(BlkTransactionState 
*common)
     }
 }
 
+static void drive_backup_clean(BlkTransactionState *common)
+{
+    DriveBackupState *state = DO_UPCAST(DriveBackupState, common, common);
+
+    if (state->aio_context) {
+        aio_context_release(state->aio_context);
+    }
+}
+
 static void abort_prepare(BlkTransactionState *common, Error **errp)
 {
     error_setg(errp, "Transaction aborted using Abort action");
@@ -1514,6 +1562,7 @@ static const BdrvActionOps actions[] = {
         .instance_size = sizeof(DriveBackupState),
         .prepare = drive_backup_prepare,
         .abort = drive_backup_abort,
+        .clean = drive_backup_clean,
     },
     [TRANSACTION_ACTION_KIND_ABORT] = {
         .instance_size = sizeof(BlkTransactionState),
@@ -1524,6 +1573,7 @@ static const BdrvActionOps actions[] = {
         .instance_size = sizeof(InternalSnapshotState),
         .prepare  = internal_snapshot_prepare,
         .abort = internal_snapshot_abort,
+        .clean = internal_snapshot_clean,
     },
 };
 
diff --git a/hw/block/dataplane/virtio-blk.c b/hw/block/dataplane/virtio-blk.c
index 1222a37..25da32a 100644
--- a/hw/block/dataplane/virtio-blk.c
+++ b/hw/block/dataplane/virtio-blk.c
@@ -198,6 +198,8 @@ void virtio_blk_data_plane_create(VirtIODevice *vdev, 
VirtIOBlkConf *conf,
     blk_op_unblock(conf->conf.blk, BLOCK_OP_TYPE_DRIVE_DEL, s->blocker);
     blk_op_unblock(conf->conf.blk, BLOCK_OP_TYPE_BACKUP_SOURCE, s->blocker);
     blk_op_unblock(conf->conf.blk, BLOCK_OP_TYPE_COMMIT, s->blocker);
+    blk_op_unblock(conf->conf.blk, BLOCK_OP_TYPE_EXTERNAL_SNAPSHOT, 
s->blocker);
+    blk_op_unblock(conf->conf.blk, BLOCK_OP_TYPE_INTERNAL_SNAPSHOT, 
s->blocker);
     blk_op_unblock(conf->conf.blk, BLOCK_OP_TYPE_MIRROR, s->blocker);
     blk_op_unblock(conf->conf.blk, BLOCK_OP_TYPE_STREAM, s->blocker);
     blk_op_unblock(conf->conf.blk, BLOCK_OP_TYPE_REPLACE, s->blocker);
-- 
2.1.0