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Date: Mon, 25 Mar 2013 08:42:57 +0100
From: Gerd Hoffmann <kraxel@redhat.com>
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Subject: Re: [Qemu-devel] Abort in monitor_puts.
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To: Luiz Capitulino <lcapitulino@redhat.com>
Cc: Anthony Liguori <aliguori@us.ibm.com>, qemu-devel <qemu-devel@nongnu.org>, =?UTF-8?B?S09OUkFEIEZyw6lkw6lyaWM=?= <fred.konrad@greensocs.com>

On 03/22/13 22:39, Luiz Capitulino wrote:
> On Fri, 22 Mar 2013 16:50:39 -0400
> Luiz Capitulino <lcapitulino@redhat.com> wrote:
>=20
>> On Fri, 22 Mar 2013 10:17:58 +0100
>> KONRAD Fr=C3=A9d=C3=A9ric <fred.konrad@greensocs.com> wrote:
>>
>>> Hi,
>>>
>>> Seems there is an issue with the current git (found by toddf on IRC).
>>>
>>> To reproduce:
>>>
>>> ./qemu-system-x86_64 --monitor stdio --nographic
>>>
>>> and put "?" it should abort.
>>>
>>> Here is the backtrace:
>>>
>>> #0  0x00007f77cd347935 in raise () from /lib64/libc.so.6
>>> #1  0x00007f77cd3490e8 in abort () from /lib64/libc.so.6
>>> #2  0x00007f77cd3406a2 in __assert_fail_base () from /lib64/libc.so.6
>>> #3  0x00007f77cd340752 in __assert_fail () from /lib64/libc.so.6
>>> #4  0x00007f77d1c1f226 in monitor_puts (mon=3D<optimized out>,
>>>      str=3D<optimized out>) at=20
>>
>> Yes, it's easy to reproduce. Bisect says:
>>
>> f628926bb423fa8a7e0b114511400ea9df38b76a is the first bad commit
>> commit f628926bb423fa8a7e0b114511400ea9df38b76a
>> Author: Gerd Hoffmann <kraxel@redhat.com>
>> Date:   Tue Mar 19 10:57:56 2013 +0100
>>
>>     fix monitor
>>    =20
>>     chardev flow control broke monitor, fix it by adding watch support.
>>    =20
>>     Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
>>
>> My impression is that monitor_puts() in being called in parallel.
>=20
> Not all.
>=20
> What's happening is that qemu_chr_fe_write() is returning < 0,
> mon->outbuf_index is not reset and is full, this causes the assert in
> monitor_puts() to trig.
>=20
> The previous version of monitor_flush() ignores errors, and everything
> works, so doing the same thing here fixes the problem :)

No, ignoring errors breaks qmp because the output isn't valid json any
more when you cut off something ...

> For some reason I'm unable to see what the error code is. Gerd, do you =
think
> the patch below is reasonable? If it's not, how should we handle errors=
 here?

No, it's not.

Ignoring the error for errno =3D EAGAIN breaks flow control.

Ignoring the error for errno !=3D EAGAIN (and maybe logging a debug
message) would be ok, but I suspect it's actually EAGAIN you get here.

Just go for a larger buffer?

cheers,
  Gerd