Re: [Qemu-devel] [PATCH 1/3] nbd-client: enter read_reply_co during init to avoid crash

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

24.08.2017 20:42, Paolo Bonzini wrote:
> On 24/08/2017 19:37, Eric Blake wrote:
>> On 08/24/2017 11:21 AM, Paolo Bonzini wrote:
>>> On 24/08/2017 17:33, Stefan Hajnoczi wrote:
>>>> This patch enters read_reply_co directly in
>>>> nbd_client_attach_aio_context().  This is safe because new_context is
>>>> acquired by the caller.  This ensures that read_reply_co reaches its
>>>> first yield point and its ctx is set up.
>>> I'm not very confident with this patch.  aio_context_acquire/release is
>>> going to go away, and this then becomes possible
>>>
>>> 	main context			new_context
>>> 	qemu_aio_coroutine_enter
>>> 					send request
>>> 					wait for reply
>>> 	read first reply
>>> 	wake coroutine
>>>
>>> where the "wake coroutine" part thinks it's running in new_context, and
>>> thus simply enters the coroutine instead of using the bottom half.
>>>
>>> But blk_co_preadv() should need the read_reply_co itself, in order to be
>>> woken up after reading the reply header.  The core issue here is that
>>> nbd_co_receive_reply was never called, I suspect.  And if it was never
>>> called, read_reply_co should not be woken up by nbd_coroutine_end.
>>>
>>> So the fix is:
>>>
>>> 1) assign NULL to s->recv_coroutine[i] when nbd_co_send_request fails
>>>
>>> 2) move this to nbd_co_receive_reply:
>>>
>>>      s->recv_coroutine[i] = NULL;
>>>
>>>      /* Kick the read_reply_co to get the next reply.  */
>>>      if (s->read_reply_co) {
>>>          aio_co_wake(s->read_reply_co);
>>>      }
>>>
>>> Does this make sense?  (Note that the read_reply_co idea actually came
>>> from you, or from my recollections of your proposed design :)).
>> How much of this overlaps with Vladimir's proposal?
>> https://lists.gnu.org/archive/html/qemu-devel/2017-08/msg00846.html
> The above should be about 15 lines added, 10 removed. :)
>
> Paolo
>
>

I'll be on vocation for the next two weeks and will continue this work 
after it.

I think we have a lot of conflicts already with my series after 
different fixes,

so I'll rebase on them all, no problem.


PS: I think recent problems and bugs in nbd are related to not very good 
separation between

block/nbd-client.c and nbd/client.c. Ideally, I think, block/nbd-client 
should not

directly access the ioc, it should just call one or two high level 
functions of

nbd/client. The big problem of this separation is CMD_READ reply - only 
real client

knows, should we read a payload or not. I have two ideas on it:

1. We can add this information to request handle, then nbd/client will 
now by handle,

   that it should read the payload.. The length of payload should be in 
handle too. It looks

  possible, as handle is 64bit when request len is 32bit.

2. Move part of NBDClientSession to nbd/client, with NBDClientRequest 
requests[MAX_NBD_REQUESTS];

   to nbd/client, to implement one public function nbd_request(state, 
*request, *reply), which will do all the

  work for block/nbd-client..


-- 
Best regards,
Vladimir