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Re: [Qemu-devel] chardev's and fd's in monitors

From: Paolo Bonzini
Subject: Re: [Qemu-devel] chardev's and fd's in monitors
Date: Wed, 19 Oct 2016 14:26:26 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0

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).

Only reads aren't, in the sense that they require an event loop so they
use that event loop for serialization.


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