Re: A bug of Monitor Chardev ?

[Peter Xu]

On Fri, May 21, 2021 at 05:56:14PM +0100, Daniel P. Berrangé wrote:
> On Fri, May 21, 2021 at 05:33:46PM +0100, Daniel P. Berrangé wrote:
> > On Fri, May 21, 2021 at 10:43:36AM -0400, Peter Xu wrote:
> > > 
> > > I think the original problem was that if qemu_chr_fe_set_handlers() is 
> > > called
> > > in main thread, it can start to race somehow within execution of the 
> > > function
> > > qemu_chr_fe_set_handlers() right after we switch context at:
> > > 
> > >     qemu_chr_be_update_read_handlers(s, context);
> > > 
> > > Then the rest code in qemu_chr_fe_set_handlers() will continue to run in 
> > > main
> > > thread for sure, but the should be running with the new iothread context, 
> > > which
> > > introduce a race condition.
> > > 
> > > Running qemu_chr_be_update_read_handlers() in BH resolves that because 
> > > then all
> > > things run in the monitor iothread only and natually serialized.
> > 
> > The first message in this thread, however, claims that it is *not*
> > in fact serialized, when using the BH. 
> > 
> > > So the new comment looks indeed not fully right, as the chr device should 
> > > be
> > > indeed within main thread context before qemu_chr_fe_set_handlers(), it's 
> > > just
> > > that the race may start right away if without BH when context switch 
> > > happens
> > > for the chr.
> > 
> > It sounds like both the comment and the code are potentially wrong.
> 
> 
> I feel like our root cause problem that the original code was trying to
> workaround, is that the chardev is "active" from the very moment it is
> created, regardless of whether the frontend is ready to use it.
> 
> IIUC, in this case the socket chardev is already listen()ing and
> accept()ing incoming clients off the network, before the monitor
> has finished configuring its hooks into the chardev. This means
> that the initial listen()/accept() I/O watches are using the
> default GMainContext, and the monitor *has* to remove them and
> put in new watches on the thread private GMainContext.
> 
> To eliminate any risk of races, we need to make it possible for the
> monitor to configure the GMainContext on the chardevs *before* any
> I/O watches are configured.
> 
> This in turn suggests that we need to split the chardev initialization
> into two phases. First we have the basic chardev creation, with object
> creation, option parsing/sanity checking, socket creation, and then
> second we have the actual activation where the I/O watches are added.

When we are still running the monitor_init() code IIUC the main thread is still
occupied so it won't be able to handle any listen()/accept() even if there's
event already, so IIUC race can only happen when main thread started the event
loop then run concurrently with the BH in the other iothread.

So, besides above split of chardev init (which sounds like a bigger
surgery)... would it also work if we let the main thread wait until that BH
executed in monitor iothread?  Then all pending listen()/accept() event will
directly be done in the monitor thread.

> 
> IOW,  qemu_chr_new() is the former and gets run from generic code in
> the main() method, or in QMP chardev_add.  A new 'qemu_chr_activate'
> method would be called by whatever frontend is using the chardev,
> after registering a custom GMainContext.
> 
> This would involve updating every single existing user of chardevs
> to add a call to qemu_chr_activate, but that's worth it to eliminate
> the race by design, rather than workaround it.
> 
> Regards,
> Daniel
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-- 
Peter Xu