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

[Paolo Bonzini]

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

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