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

From: Fam Zheng
Subject: [Qemu-devel] [PATCH 0/1] block: Workaround for the iotests errors
Date: Fri, 24 Nov 2017 01:57:46 +0800

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
address@hidden) 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

Note this doesn't fix the hanging 056, which I haven't debugged yet.


Fam Zheng (1):
  block: Don't poll for drain end

 block/io.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)


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