Re: [PATCH 1/3] block/nbd: allow drain during reconnect attempt

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

23.07.2020 21:47, Eric Blake wrote:
> On 7/20/20 4:00 AM, Vladimir Sementsov-Ogievskiy wrote:
>> It should be to reenter qio_channel_yield() on io/channel read/write
>> path, so it's safe to reduce in_flight and allow attaching new aio
>> context. And no problem to allow drain itself: connection attempt is
>> not a guest request. Moreover, if remote server is down, we can hang
>> in negotiation, blocking drain section and provoking a dead lock.
>>
>> How to reproduce the dead lock:
>>
> 
> I tried to reproduce this; but in the several minutes it has taken me to write this email, it still has not hung.  Still, your stack trace is fairly good evidence of the problem, where adding a temporary sleep or running it under gdb with a breakpoint can probably make reproduction easier.

I've tried to make a reproduce, adding temporary BDRV_POLL_WHILE, but I failed.

One time, it reproduced for me after 4000 iterations, but other times a lot earlier.

It may help to start several qemu-io loop in parallel.

Also, iotest 83 for -nbd hangs sometimes for me as well.

-- 
Best regards,
Vladimir