From: Alberto Garcia <berto@igalia.com>
To: Anton Nefedov <anton.nefedov@virtuozzo.com>, Fam Zheng <famz@redhat.com>
Cc: qemu-devel@nongnu.org, kwolf@redhat.com, qemu-block@nongnu.org,
mreitz@redhat.com, John Snow <jsnow@redhat.com>
Subject: Re: [Qemu-devel] [Qemu-block] segfault in parallel blockjobs (iotest 30)
Date: Tue, 21 Nov 2017 13:51:34 +0100 [thread overview]
Message-ID: <w51zi7fbxrd.fsf@maestria.local.igalia.com> (raw)
In-Reply-To: <41e905e4-0c2a-fad3-09a6-4959f04fe546@virtuozzo.com>
On Thu 16 Nov 2017 05:09:59 PM CET, Anton Nefedov wrote:
>>>> I have the impression that one major source of headaches is the
>>>> fact that the reopen queue contains nodes that don't need to be
>>>> reopened at all. Ideally this should be detected early on in
>>>> bdrv_reopen_queue(), so there's no chance that the queue contains
>>>> nodes used by a different block job. If we had that then op
>>>> blockers should be enough to prevent these things. Or am I missing
>>>> something?
>>>>
>>> After applying Max's patch I tried the similar approach; that is
>>> keep BDSes referenced while they are in the reopen queue. Now I get
>>> the stream job hanging. Somehow one blk_root_drained_begin() is not
>>> paired with blk_root_drained_end(). So the job stays paused.
>>
>> I can see this if I apply Max's patch and keep refs to BDSs in the
>> reopen queue:
>>
>> #0 block_job_pause (...) at blockjob.c:130
>> #1 0x000055c143cb586d in block_job_drained_begin (...) at blockjob.c:227
>> #2 0x000055c143d08067 in blk_set_dev_ops (...) at block/block-backend.c:887
>> #3 0x000055c143cb69db in block_job_create (...) at blockjob.c:678
>> #4 0x000055c143d17c0c in mirror_start_job (...) at block/mirror.c:1177
>>
>> There's a ops->drained_begin(opaque) call in blk_set_dev_ops() that
>> doesn't seem to be paired.
>
> My understanding for now is
>
> 1. in bdrv_drain_all_begin(), BDS gets drained with
> bdrv_parent_drained_begin(), all the parents get
> blk_root_drained_begin(), pause their jobs.
> 2. one of the jobs finishes and deletes BB.
> 3. in bdrv_drain_all_end(), bdrv_parent_drained_end() is never
> called because even though BDS still exists (referenced in the
> queue), it cannot be accessed as bdrv_next() takes BDS from the
> global BB list (and the corresponding BB is deleted).
Yes, I was debugging this and I got to a similar conclusion. With
test_stream_commit from iotest 030 I can see that
1. the block-stream job is created first, then stream_run begins and
starts copying the data.
2. block-commit starts and tries to reopen the top image in
read-write mode. This pauses the stream block job and drains all
BDSs.
3. The draining causes the stream job to finish, it is deferred to
the main loop, then stream_complete finishes and unrefs the block
job, deleting it. At the point of deletion the pause count was
still > 0 (because of step (2))
> Max's patch v1 could have helped:
> http://lists.nongnu.org/archive/html/qemu-devel/2017-11/msg01835.html
This doesn't solve the problem.
> Or, perhaps another approach, keep BlockJob referenced while it is
> paused (by block_job_pause/resume_all()). That should prevent it from
> deleting the BB.
Yes, I tried this and it actually solves the issue. But I still think
that the problem is that block jobs are allowed to finish when they are
paused.
Adding block_job_pause_point(&s->common) at the end of stream_run()
fixes the problem too.
Berto
next prev parent reply other threads:[~2017-11-21 12:51 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-07 16:19 [Qemu-devel] segfault in parallel blockjobs (iotest 30) Anton Nefedov
2017-11-08 13:58 ` [Qemu-devel] [Qemu-block] " Alberto Garcia
2017-11-08 14:45 ` Alberto Garcia
2017-11-08 15:50 ` Anton Nefedov
2017-11-09 6:05 ` Fam Zheng
2017-11-09 10:03 ` Alberto Garcia
2017-11-09 16:26 ` Alberto Garcia
2017-11-10 3:02 ` Fam Zheng
2017-11-15 15:42 ` Alberto Garcia
2017-11-15 16:31 ` Anton Nefedov
2017-11-16 15:54 ` Alberto Garcia
2017-11-16 16:09 ` Anton Nefedov
2017-11-21 12:51 ` Alberto Garcia [this message]
2017-11-21 15:18 ` Anton Nefedov
2017-11-21 15:31 ` Alberto Garcia
2017-11-22 0:13 ` John Snow
2017-11-22 12:55 ` Alberto Garcia
2017-11-22 15:58 ` John Snow
2017-11-16 21:56 ` John Snow
2017-11-17 16:16 ` Alberto Garcia
2017-11-22 15:05 ` Alberto Garcia
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=w51zi7fbxrd.fsf@maestria.local.igalia.com \
--to=berto@igalia.com \
--cc=anton.nefedov@virtuozzo.com \
--cc=famz@redhat.com \
--cc=jsnow@redhat.com \
--cc=kwolf@redhat.com \
--cc=mreitz@redhat.com \
--cc=qemu-block@nongnu.org \
--cc=qemu-devel@nongnu.org \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.