block-backend: Fix blk_new_open() for iothreads
This fixes blk_new_open() to not assume that bs is in the main context.
In particular, the BlockBackend must be created with the right
AioContext because it will refuse to move to a different context
afterwards. (blk->allow_aio_context_change is false.)
Use this opportunity to use blk_insert_bs() instead of duplicating the
bdrv_root_attach_child() call. This is consistent with what
blk_new_with_bs() does. Add comments to document the locking rules.
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Message-Id: <20230525124713.401149-5-kwolf@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
diff --git a/block/block-backend.c b/block/block-backend.c
index 1d89fab..dde60e0 100644
--- a/block/block-backend.c
+++ b/block/block-backend.c
@@ -389,6 +389,8 @@
* Both sets of permissions can be changed later using blk_set_perm().
*
* Return the new BlockBackend on success, null on failure.
+ *
+ * Callers must hold the AioContext lock of @bs.
*/
BlockBackend *blk_new_with_bs(BlockDriverState *bs, uint64_t perm,
uint64_t shared_perm, Error **errp)
@@ -406,11 +408,15 @@
/*
* Creates a new BlockBackend, opens a new BlockDriverState, and connects both.
- * The new BlockBackend is in the main AioContext.
+ * By default, the new BlockBackend is in the main AioContext, but if the
+ * parameters connect it with any existing node in a different AioContext, it
+ * may end up there instead.
*
* Just as with bdrv_open(), after having called this function the reference to
* @options belongs to the block layer (even on failure).
*
+ * Called without holding an AioContext lock.
+ *
* TODO: Remove @filename and @flags; it should be possible to specify a whole
* BDS tree just by specifying the @options QDict (or @reference,
* alternatively). At the time of adding this function, this is not possible,
@@ -422,6 +428,7 @@
{
BlockBackend *blk;
BlockDriverState *bs;
+ AioContext *ctx;
uint64_t perm = 0;
uint64_t shared = BLK_PERM_ALL;
@@ -451,18 +458,24 @@
shared = BLK_PERM_CONSISTENT_READ | BLK_PERM_WRITE_UNCHANGED;
}
- blk = blk_new(qemu_get_aio_context(), perm, shared);
aio_context_acquire(qemu_get_aio_context());
bs = bdrv_open(filename, reference, options, flags, errp);
aio_context_release(qemu_get_aio_context());
if (!bs) {
- blk_unref(blk);
return NULL;
}
- blk->root = bdrv_root_attach_child(bs, "root", &child_root,
- BDRV_CHILD_FILTERED | BDRV_CHILD_PRIMARY,
- perm, shared, blk, errp);
+ /* bdrv_open() could have moved bs to a different AioContext */
+ ctx = bdrv_get_aio_context(bs);
+ blk = blk_new(bdrv_get_aio_context(bs), perm, shared);
+ blk->perm = perm;
+ blk->shared_perm = shared;
+
+ aio_context_acquire(ctx);
+ blk_insert_bs(blk, bs, errp);
+ bdrv_unref(bs);
+ aio_context_release(ctx);
+
if (!blk->root) {
blk_unref(blk);
return NULL;
@@ -903,6 +916,8 @@
/*
* Associates a new BlockDriverState with @blk.
+ *
+ * Callers must hold the AioContext lock of @bs.
*/
int blk_insert_bs(BlockBackend *blk, BlockDriverState *bs, Error **errp)
{
