Jeff's block job patch made the latent drain bug visible, and I find this patch, which by itself also makes some sense, can hide it again. :) With it applied we are at least back to the ground where patchew's iotests (make docker-test-block@fedora) can pass.
The real bug is that in the middle of bdrv_parent_drained_end(), bs's parent list changes. One drained_end call before the mirror_exit() already did one blk_root_drained_end(), a second drained_end on an updated parent node can do another same blk_root_drained_end(), making it unbalanced with blk_root_drained_begin(). This is shown by the following three backtraces as captured by rr with a crashed "qemu-img commit", essentially the same as in the failed iotest 020: * Backtrace 1, where drain begins: (rr) bt * Backtrace 2, in the early phase of bdrv_parent_drained_end(), before mirror_exit happend: (rr) bt * Backtrace 3, in a later phase of the same bdrv_parent_drained_end(), after mirror_exit() which changed the node graph: (rr) bt IMO we should rethink bdrv_parent_drained_begin/end to avoid such complications and maybe in the long term get rid of the nested BDRV_POLL_WHILE() if possible. It's late for me so I'm posting the patch anyway in case we could use it for -rc3. Note this doesn't fix the hanging 056, which I haven't debugged yet. Fam Fam Zheng (1): block: Don't poll for drain end block/io.c | 8 +++++--- 1 file changed, 5 insertions(+), 3 deletions(-) -- 2.14.3
