Re: [Qemu-devel] [PATCH v7 0/7] push mmio dispatch out of big lock

[Paolo Bonzini <pbonzini@redhat.com>]

Il 06/05/2013 12:56, Jan Kiszka ha scritto:
>> The problem is that even if I/O for a region is supposed to happen
>> within the BQL, lookup can happen outside the BQL.  Lookup will use the
>> region even if it is just to discard it:
>>
>>            VCPU thread (under BQL)              device thread
>>  --------------------------------------------------------------------------------------
>>                                                 flatview_ref
>>                                                 memory_region_find returns d->mr
>>                                                 memory_region_ref(d->mr) /* nop */
>>            qdev_free(d)
>>              object_unparent(d)
>>                unrealize(d)
>>                  memory_region_del_subregion(d->mr)
>>                    FlatView updated, d->mr not in the new view
>>
>>                                                 flatview_unref
>>                                                   memory_region_unref(d->mr)
>>                                                     object_unref(d)
>>                                                       free(d)
>>                                                 if (!d->mr->is_ram) {        /* BAD! */
>>                                                   memory_region_unref(d->mr) /* nop */
>>                                                   return error
>>                                                 }
>>
>>
>> Here, the memory region is dereferenced *before* we know that it is BQL-free
>> (in fact, exactly to ascertain whether it is BQL-free).
> 
> Both flatview update and lookup *plus* locking type evaluation (i.e.
> memory region dereferencing) always happen under the address space lock.
> See Pingfan's patch.

That's true of address_space_rw/map, but I don't think it holds for
memory_region_find.

Paolo