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Re: [Qemu-block] [PATCH] nbd/server: attach client channel to the export

From: Eric Blake
Subject: Re: [Qemu-block] [PATCH] nbd/server: attach client channel to the export's AioContext
Date: Wed, 11 Sep 2019 17:03:52 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.8.0

On 9/11/19 4:33 PM, Eric Blake wrote:

> I tried to test this patch, but even with it applied, I still got an
> aio-context crasher by attempting an nbd-server-start, nbd-server-add,
> nbd-server-stop (intentionally skipping the nbd-server-remove step) on a
> domain using iothreads, with a backtrace of:

The crash also reproduces with nbd-server-remove instead of
nbd-server-stop.  Both QMP commands eventually call into the
blk_remove_bs() that calls the fatal blk_drain().  So libvirt can't work
around the bug merely by adding an nbd-server-remove step.

> #0  0x00007ff09d070e35 in raise () from target:/lib64/libc.so.6
> #1  0x00007ff09d05b895 in abort () from target:/lib64/libc.so.6
> #2  0x000055dd03b9ab86 in error_exit (err=1, msg=0x55dd03d59fb0
> <__func__.15769> "qemu_mutex_unlock_impl")
>     at util/qemu-thread-posix.c:36
> #3  0x000055dd03b9adcf in qemu_mutex_unlock_impl (mutex=0x55dd062d5090,
> file=0x55dd03d59041 "util/async.c",
>     line=523) at util/qemu-thread-posix.c:96
> #4  0x000055dd03b93433 in aio_context_release (ctx=0x55dd062d5030) at
> util/async.c:523
> #5  0x000055dd03ac421b in bdrv_do_drained_begin (bs=0x55dd0673a2d0,
> recursive=false, parent=0x0,
>     ignore_bds_parents=false, poll=true) at block/io.c:428
> #6  0x000055dd03ac4299 in bdrv_drained_begin (bs=0x55dd0673a2d0) at
> block/io.c:434
> #7  0x000055dd03aafb54 in blk_drain (blk=0x55dd06a3ec40) at
> block/block-backend.c:1605
> #8  0x000055dd03aae054 in blk_remove_bs (blk=0x55dd06a3ec40) at
> block/block-backend.c:800

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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