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
