Re: [Qemu-devel] [RFC PATCH 0/2] establish nesting rule of BQL vs cpu-exclusive

[Roman Kagan]

On Thu, May 23, 2019 at 12:31:16PM +0100, Alex Bennée wrote:
> 
> Roman Kagan <address@hidden> writes:
> 
> > I came across the following AB-BA deadlock:
> >
> >     vCPU thread                             main thread
> >     -----------                             -----------
> > async_safe_run_on_cpu(self,
> >                       async_synic_update)
> > ...                                         [cpu hot-add]
> > process_queued_cpu_work()
> >   qemu_mutex_unlock_iothread()
> >                                             [grab BQL]
> >   start_exclusive()                         cpu_list_add()
> >   async_synic_update()                        finish_safe_work()
> >     qemu_mutex_lock_iothread()                  cpu_exec_start()
> >
> > ATM async_synic_update seems to be the only async safe work item that
> > grabs BQL.  However it isn't quite obvious that it shouldn't; in the
> > past there were more examples of this (e.g.
> > memory_region_do_invalidate_mmio_ptr).
> >
> > It looks like the problem is generally in the lack of the nesting rule
> > for cpu-exclusive sections against BQL, so I thought I would try to
> > address that.  This patchset is my feeble attempt at this; I'm not sure
> > I fully comprehend all the consequences (rather, I'm sure I don't) hence
> > RFC.
> 
> Hmm I think this is an area touched by:
> 
>   Subject: [PATCH v7 00/73] per-CPU locks
>   Date: Mon,  4 Mar 2019 13:17:00 -0500
>   Message-Id: <address@hidden>
> 
> which has stalled on it's path into the tree. Last time I checked it
> explicitly handled the concept of work that needed the BQL and work that
> didn't.

I'm still trying to get my head around that patchset, but it looks like
it changes nothing in regards to cpu-exclusive sections and safe work,
so it doesn't make the problem go.

> How do you trigger your deadlock? Just hot-pluging CPUs?

Yes.  The window is pretty narrow so I only saw it once although this
test (where the vms are started and stopped and the cpus are plugged in
and out) is in our test loop for quite a bit (probably 2+ years).

Roman.