Re: [Qemu-devel] [PATCH 2/5] exec.c: use refcnt to protect device during dispatching

[liu ping fan]

On Wed, Jul 25, 2012 at 5:18 PM, Paolo Bonzini <address@hidden> wrote:
> Il 25/07/2012 10:12, liu ping fan ha scritto:
>>>> >> +        qemu_rwlock_rdlock_devtree();
>>>> >>          section = phys_page_find(page >> TARGET_PAGE_BITS);
>>>> >> +        if (!(memory_region_is_ram(section->mr) ||
>>>> >> +            memory_region_is_romd(section->mr)) && !is_write) {
>>>> >> +            bk = get_backend(section->mr, addr);
>>>> >> +            object_ref(bk);
>>>> >> +        } else if (!memory_region_is_ram(section->mr) && is_write) {
>>>> >> +            bk = get_backend(section->mr, addr);
>>>> >> +            object_ref(bk);
>>>> >> +        }
>>>> >> +        qemu_rwlock_unlock_devtree();
>>>> >>
>>>> >>          if (is_write) {
>>>> >>              if (!memory_region_is_ram(section->mr)) {
>>>> >> @@ -3426,6 +3462,7 @@ void cpu_physical_memory_rw(target_phys_addr_t 
>>>> >> addr, uint8_t *buf,
>>>> >>                      io_mem_write(section->mr, addr1, val, 1);
>>>> >>                      l = 1;
>>>> >>                  }
>>>> >> +                object_unref(bk);
>>> >
>>> > Currently object_ref()/object_unref() are not atomic.  Will you send
>> We obey the rule:
>>            rdlock->search->ref_get,
>>            wrlock->remove ->ref_put
>> So can  it causes problem if object_ref()/object_unref()  are not atomic?
>
> Yes, two CPUs can perform object_ref at the same time.
>
> You can find a header file for atomic operations here:
> https://github.com/bonzini/qemu/commit/atomics.patch
>
Got it, thanks

> Paolo