Re: [Qemu-devel] [PATCH 0/1] block: Workaround for the iotests errors

[Kevin Wolf <kwolf@redhat.com>]

Am 23.11.2017 um 18:57 hat Fam Zheng geschrieben:
> 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.

Maybe the backtraces would help me understand the problem if they were
actually there. :-)

Kevin