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From: Paolo Bonzini <pbonzini@redhat.com>
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Date: Wed, 19 Oct 2016 22:51:07 +0200
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Subject: Re: [Qemu-devel] chardev's and fd's in monitors
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To: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
Cc: qemu-devel@nongnu.org, armbru@redhat.com



On 19/10/2016 19:01, Dr. David Alan Gilbert wrote:
> * Paolo Bonzini (pbonzini@redhat.com) wrote:
>>
>>
>> On 18/10/2016 16:01, Daniel P. Berrange wrote:
>>>>>
>>>>> I already use error_report's in places in migration threads of various
>>>>> types; I'm not sure if that's a problem.
>>> Unless those places are protected by the big qemu lock, that sounds
>>> not good. error_report calls into error_vprintf which checks the
>>> 'cur_mon' global "Monitor" pointer. This variable is updated at
>>> runtime - eg in qmp_human_monitor_command(), monitor_qmp_read(),
>>> monitor_read(), etc. So if migration threads outside the BQL are
>>> calling error_report() that could well cause problems. If you
>>> are lucky messages will merely end up going to stderr instead of
>>> the monitor, but in worst case I wouldn't be surprised if there
>>> is a crash possibility in some race conditions.
>>
>> Writes to chardevs *are* thread-safe (assuming qio_channel_create_watch
>> is thread-safe; it seems to be).
> 
> Hmm that's useful (although it doesn't solve error_report because error_vprintf
> is racy itself).

How is it racy?  Because of the case where cur_mon changes under the
feet of error_vprintf?  I guess that can be ignored for now (just a TODO
comment will do).

> How deadlock safe is it?  In particular imagine I've got another
> thread which is doing:
>   take bql
>      <many layers of stuff>
>      write to a chardev
>   release bql
> 
>   if that chardev is registered on the main thread, will that
> deadlock?

No, it won't.  Since your usecase is about the monitor, note that the
monitor does its own buffering; if the nonblocking write leaves stuff in
the buffer, the monitor will process the watch only after the BQL is
released by the thread.  However, there's no deadlock.

Paolo

>> Only reads aren't, in the sense that they require an event loop so they
>> use that event loop for serialization.
> 
> Hmm OK.
> 
> Dave
> 
>>
>> Paolo
> --
> Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> 
>