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Re: [Qemu-devel] [PATCH] do not call monitor_resume() from migrate_fd_pu
From: |
Michael Tokarev |
Subject: |
Re: [Qemu-devel] [PATCH] do not call monitor_resume() from migrate_fd_put_buffer() error path |
Date: |
Fri, 05 Aug 2011 00:00:31 +0400 |
User-agent: |
Mozilla/5.0 (X11; U; Linux i686 (x86_64); en-US; rv:1.9.1.16) Gecko/20110704 Icedove/3.0.11 |
04.08.2011 23:52, Michael Tokarev пишет:
> 04.08.2011 23:24, Luiz Capitulino пишет:
>> On Wed, 3 Aug 2011 18:51:44 +0400
>> Michael Tokarev <address@hidden> wrote:
>>
>>> If we do, it results in double monitor_resume() (second being called
>>> from migrate_fd_cleanup() anyway) and monitor suspend count becoming
>>> negative.
>>
>> Are you hitting an specific issue or did you find this by code inspection?
>
> The specific case I'm talking about can trivially be triggered.
> Run migrate "exec:nosuchfile" and your monitor will be stuck
> forever.
>
>> IIRC, I asked Marcelo to add the monitor_resume() call in the fix for
>> e447b1a603 because the monitor wasn't being resumed in some cases. Don't
>> remember which though, do you Marcelo?
>
> And this (e447b1a603) is actually exactly the same thing: when the
> child terminates before qemu completes sending stuff to it.
>
>> I see two possibilities here:
>>
>> 1. After e447b1a603 there was some change that made the monitor_resume()
>> call
>> in migrate_fd_put_buffer() unnecessary
>>
>> 2. We're calling it in the wrong place
>>
>> Taking a quick look at the code I see that migrate_fd_cleanup() doesn't
>> seem to be called when qemu_savevm_state_iterate() fails, for example.
>
> Yes indeed, but that will lead to a havoc in other places, like
> leaving callback notifier registered even after migration gets
> cancelled, or keeping the file open.
>
> Note that qemu_savevm_state_iterate() gets called from a callback
> routine.
>
> What I observe here is that when the exec: process exits prematurely,
> qemu will continue writing stuff to pipe up till migrate_fd_put_buffer()
> hits error (EPIPE iirc), at which point we'll call mon->resume() twice.
> The same happens in case of tcp migration when the other end closes
> the connection.
>
> I don't know when qemu_savevm_state_iterate() may fail, or why
> qemu_file_has_error() in there does not return true.
>
> Maybe the problem here is much more severe actually, -- that's why
> I initially asked what it is all about, -- before offering the patch.
This looks like a race condition: which of the two places
will detect the error first.
Sometimes I see this (-monitor stdio):
(qemu) migrate "exec:nothing"
sh: nothing: not found
(qemu)
and after this, monitor is stuck as I described. But sometimes
I see this:
(qemu) migrate "exec:nothing"
sh: nothing: not found
(qemu)
And it continues working. Note the extra newline.
If the error gets detected sooner (like in case of
exec:nothing, or even better, after removing
/bin/sh so that system(3) will fail), the chance
to have stuck monitor is much higher. If the
error gets detected later, it will survive most
likely. Or something like that anyway.
/mjt