block: .bdrv_open is non-coroutine and unlocked

Drivers were a bit confused about whether .bdrv_open can run in a
coroutine and whether or not it holds a graph lock.

It cannot keep a graph lock from the caller across the whole function
because it both changes the graph (requires a writer lock) and does I/O
(requires a reader lock). Therefore, it should take these locks
internally as needed.

The functions used to be called in coroutine context during image
creation. This was buggy for other reasons, and as of commit 32192301,
all block drivers go through no_co_wrappers. So it is not called in
coroutine context any more.

Fix qcow2 and qed to work with the correct assumptions: The graph lock
needs to be taken internally instead of just assuming it's already
there, and the coroutine path is dead code that can be removed.

Signed-off-by: Kevin Wolf <kwolf@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-Id: <20230504115750.54437-9-kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

1 files changed, 6 insertions, 9 deletions

diff --git a/block/qcow2.c b/block/qcow2.c
index 01742b3ebe..5bde3b8401 100644
--- a/block/qcow2.c
+++ b/block/qcow2.c
@@ -1891,7 +1891,7 @@ static void coroutine_fn qcow2_open_entry(void *opaque)
     QCow2OpenCo *qoc = opaque;
     BDRVQcow2State *s = qoc->bs->opaque;
 
-    assume_graph_lock(); /* FIXME */
+    GRAPH_RDLOCK_GUARD();
 
     qemu_co_mutex_lock(&s->lock);
     qoc->ret = qcow2_do_open(qoc->bs, qoc->options, qoc->flags, true,
@@ -1920,14 +1920,11 @@ static int qcow2_open(BlockDriverState *bs, QDict *options, int flags,
     /* Initialise locks */
     qemu_co_mutex_init(&s->lock);
 
-    if (qemu_in_coroutine()) {
-        /* From bdrv_co_create.  */
-        qcow2_open_entry(&qoc);
-    } else {
-        assert(qemu_get_current_aio_context() == qemu_get_aio_context());
-        qemu_coroutine_enter(qemu_coroutine_create(qcow2_open_entry, &qoc));
-        BDRV_POLL_WHILE(bs, qoc.ret == -EINPROGRESS);
-    }
+    assert(!qemu_in_coroutine());
+    assert(qemu_get_current_aio_context() == qemu_get_aio_context());
+    qemu_coroutine_enter(qemu_coroutine_create(qcow2_open_entry, &qoc));
+    BDRV_POLL_WHILE(bs, qoc.ret == -EINPROGRESS);
+
     return qoc.ret;
 }