Re: [Qemu-devel] [PATCH] monitor: Protect outbuf from concurrent access

[Daniel P. Berrange]

On Thu, Sep 01, 2011 at 08:34:35PM -0500, Anthony Liguori wrote:
> On 09/01/2011 02:35 PM, Luiz Capitulino wrote:
> >Sometimes, when having lots of VMs running on a RHEV host and the user
> >attempts to close a SPICE window, libvirt will get corrupted json from
> >QEMU.
> >
> >After some investigation, I found out that the problem is that different
> >SPICE threads are calling monitor functions (such as
> >monitor_protocol_event()) in parallel which causes concurrent access
> >to the monitor's internal buffer outbuf[].
> >
> >This fixes the problem by protecting accesses to outbuf[] with a mutex.
> >
> >Honestly speaking, I'm not completely sure this the best thing to do
> >because the monitor itself and other qemu subsystems are not thread safe,
> >so having subsystems like SPICE assuming the contrary seems a bit
> >catastrophic to me...
> >
> >Anyways, this commit fixes the problem at hand.
> 
> Nack.
> 
> This is absolutely a Spice bug.  Spice should not be calling into
> QEMU code from multiple threads.  It should only call into QEMU code
> while it's holding the qemu_mutex.
> 
> The right way to fix this is probably to make all of the
> SpiceCoreInterface callbacks simply write to a file descriptor which
> can then wake up QEMU to do the operation on behalf of it.   It's
> ugly but the libspice interface is far too tied to QEMU internals in
> the first place which is the root of the problem.

This feels like a rather short-term approach to fixing the problem
to me. As QEMU becomes increasingly multi-threaded, there is high
liklihood that we'll get other code in QEMU which wants to use the
monitor from multiple threads. The monitor code in QEMU is fairly
well isolated & thus comparatively easy to make threadsafe, so I
don't see why we wouldn't want todo that & avoid any chance of this
type of problem recurring in the future.

IMHO, "fixing" SPICE is not fixing the bug at all, it is just removing
the trigger of the bug in the monitor.

Regards,
Daniel