zio_done panic in 10.3

Andriy Gapon avg at FreeBSD.org
Wed Nov 22 15:29:07 UTC 2017

On 22/11/2017 16:40, Youzhong Yang wrote:
> Hi Andriy,
> This is nice! I am 100% sure it's exactly the same issue I experienced and then
> reported to illumos mailing list. In all the crash dumps zio->io_done =
> l2arc_read_done, so I thought the crash must be related to L2ARC. Once I set
> secondarycache=metadata, the frequency of crash went from one per 2 days down to
> one per week. I've been puzzled by what could have caused a zio being destroyed
> while there's still child zio. Your explanation definitely makes sense!

Oh, I now recall seeing your report:
I remember that it raised my interest, but then I forgot about it and didn't
correlate it with the latest reports.

> By the way, is there a FreeBSD bug report or an illumos bug number tracking this
> issue? I would be more than happy to create one if needed, and also test your
> potential fix here in our environment.

I am not aware of any existing bug report.
It would be great if you could open one [ or two :-) ]
If you open an illumos issue, please also add George Wilson as a watcher.
I think that George is also interested in fixing this issue and he knows the
relevant code better than me.

Thank you!

> On Tue, Nov 21, 2017 at 3:46 PM, Andriy Gapon <avg at freebsd.org
> <mailto:avg at freebsd.org>> wrote:
>     On 21/11/2017 21:30, Shiva Bhanujan wrote:
>     > it did get compressed to 0.5G - still too big to send via email.  I did send some more debug information by running kgdb on the core file to Andriy, and I'm waiting for any analysis that he might provide.
>     Yes, kgdb-over-email turned out to be a far more efficient compression :-)
>     I already have an analysis based on the information provided by Shiva and by
>     another user who has the same problem and contacted me privately.
>     I am discussing possible ways to fix the problem with George Wilson who was very
>     kind to double-check the analysis, complete it and suggest possible fixes.
>     A short version is that dbuf_prefetch and dbuf_prefetch_indirect_done functions
>     chain new zio-s under the same parent zio (a completion of one child zio may
>     create another child zio).  They do it using arc_read which can create either a
>     logical zio in most cases or a vdev zio for a read from a cache device (2arc).
>     zio_done() has a check for the completion of a parent zio's children but that
>     check is not completely safe and can be broken by the pattern that dbuf_prefetch
>     can create.  So, under some specific circumstances the parent zio may complete
>     and get destroyed while there is a child zio.
>     I believe this problem to be rather rare, but there could be configurations and
>     workloads where it's triggered more often.
>     The problem does not happen if there are no cache devices.
>     > From: Conrad Meyer [cem at freebsd.org <mailto:cem at freebsd.org>]
>     >
>     > Sent: Tuesday, November 21, 2017 9:04 AM
>     >
>     > To: Shiva Bhanujan
>     >
>     > Cc: Andriy Gapon; freebsd-fs at freebsd.org <mailto:freebsd-fs at freebsd.org>
>     >
>     > Subject: Re: zio_done panic in 10.3
>     >
>     >
>     >
>     >
>     >
>     >
>     >
>     > Have you tried compressing it with e.g. xz or zstd?
>     >
>     > --
>     Andriy Gapon
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Andriy Gapon

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