Removing a drive with drive_del while it is being used to run an I/O intensive workload can cause QEMU to crash.

An AIO flush can yield at some point:

blk_aio_flush_entry() blk_co_flush(blk) bdrv_co_flush(blk->root->bs) ... qemu_coroutine_yield()

and let the HMP command to run, free blk->root and give control back to the AIO flush:

hmp_drive_del() blk_remove_bs() bdrv_root_unref_child(blk->root) child_bs = blk->root->bs bdrv_detach_child(blk->root) bdrv_replace_child(blk->root, NULL) blk->root->bs = NULL g_free(blk->root) <============== blk->root becomes stale bdrv_unref(child_bs) bdrv_delete(child_bs) bdrv_close() bdrv_drained_begin() bdrv_do_drained_begin() bdrv_drain_recurse() aio_poll() ... qemu_coroutine_switch()

and the AIO flush completion ends up dereferencing blk->root:

blk_aio_complete() scsi_aio_complete() blk_get_aio_context(blk) bs = blk_bs(blk) ie, bs = blk->root ? blk->root->bs : NULL ^^^^^ stale

The problem is that we should avoid making block driver graph changes while we have in-flight requests. This patch hence adds a drained section to bdrv_detach_child(), so that we're sure all requests have been drained when blk->root is freed.

Signed-off-by: Greg Kurz groug@kaod.org --- v2: - drain I/O requests when detaching the BDS (Stefan, Paolo) --- block.c | 4 ++++ 1 file changed, 4 insertions(+)

diff --git a/block.c b/block.c index 676e57f5623a..fc9379439883 100644 --- a/block.c +++ b/block.c @@ -2127,12 +2127,16 @@ BdrvChild *bdrv_attach_child(BlockDriverState *parent_bs,

static void bdrv_detach_child(BdrvChild *child) { + BlockDriverState *child_bs = child->bs; + if (child->next.le_prev) { QLIST_REMOVE(child, next); child->next.le_prev = NULL; }

+ bdrv_drained_begin(child_bs); bdrv_replace_child(child, NULL); + bdrv_drained_end(child_bs);

g_free(child->name); g_free(child);