Re: [Qemu-devel] [PATCH 1/3] colo-compare: serialize compare thread's initialization with main thread

[Jason Wang <jasowang@redhat.com>]

On 2017年04月25日 17:59, Hailiang Zhang wrote:
> On 2017/4/25 16:41, Jason Wang wrote:
>>
>> On 2017年04月24日 14:03, Hailiang Zhang wrote:
>>> On 2017/4/24 12:10, Jason Wang wrote:
>>>> On 2017年04月20日 15:46, zhanghailiang wrote:
>>>>> We call qemu_chr_fe_set_handlers() in colo-compare thread, it is used
>>>>> to detach watched fd from default main context, so it has chance to
>>>>> handle the same watched fd with main thread concurrently, which will
>>>>> trigger an error report:
>>>>> "qemu-char.c:918: io_watch_poll_finalize: Assertion `iwp->src ==
>>>>> ((void *)0)' failed."
>>>> Anyway to prevent fd from being handled by main thread before creating
>>>> colo thread? Using semaphore seems not elegant.
>>> So how about calling qemu_mutex_lock_iothread() before
>>> qemu_chr_fe_set_handlers() ?
>> Looks better, but I needs more information e.g how main thread can 
>> touch it?
>
> Hmm, this happened quite occasionally, and we didn't catch the first 
> place (backtrace)
> of removing fd from been watched, but  from the codes logic, we found 
> there should
> be such possible cases:
> tcp_chr_write (Or tcp_chr_read/tcp_chr_sync_read/chr_disconnect)
>  ->tcp_chr_disconnect (Or char_socket_finalize)
>     ->tcp_chr_free_connection
>       -> remove_fd_in_watch(chr);
>
> Anyway, it needs the protection from been freed twice.
>
> Thanks,
> Hailiang

Still a little bit confused. The question is how could main thread still 
call tcp_chr_write or other in the above case?

Thanks

>> Thanks
>>
>> .
>>
>
>