Re: [Qemu-devel] Abort in monitor_puts.

[Gerd Hoffmann]

On 03/22/13 22:39, Luiz Capitulino wrote:
> On Fri, 22 Mar 2013 16:50:39 -0400
> Luiz Capitulino <address@hidden> wrote:
> 
>> On Fri, 22 Mar 2013 10:17:58 +0100
>> KONRAD Frédéric <address@hidden> 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=<optimized out>,
>>>      str=<optimized out>) at 
>>
>> Yes, it's easy to reproduce. Bisect says:
>>
>> f628926bb423fa8a7e0b114511400ea9df38b76a is the first bad commit
>> commit f628926bb423fa8a7e0b114511400ea9df38b76a
>> Author: Gerd Hoffmann <address@hidden>
>> Date:   Tue Mar 19 10:57:56 2013 +0100
>>
>>     fix monitor
>>     
>>     chardev flow control broke monitor, fix it by adding watch support.
>>     
>>     Signed-off-by: Anthony Liguori <address@hidden>
>>
>> My impression is that monitor_puts() in being called in parallel.
> 
> Not all.
> 
> 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.
> 
> 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 = EAGAIN breaks flow control.

Ignoring the error for errno != 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