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

[Alex Bennée]

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.

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

>
> Roman Kagan (2):
>   cpus-common: nuke finish_safe_work
>   cpus-common: assert BQL nesting within cpu-exclusive sections
>
>  cpus-common.c | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)


--
Alex Bennée