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

[Roman Kagan <rkagan@virtuozzo.com>]

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.

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(-)

-- 
2.21.0