Re: [Qemu-devel] [PATCH] virtio-blk: dataplane: release AioContext before blk_set_aio_context

[Kevin Wolf]

Am 01.03.2019 um 14:47 hat Sergio Lopez geschrieben:
> >> Otherwise, we can simply add an extra condition at
> >> child_job_drained_poll(), before the drv->drained_poll(), to return
> >> true if the job isn't yet paused.
> >
> > Yes, I think something like this is this right fix.
> 
> Fixing this has uncovered another issue also triggered by issuing
> 'block_commit' and 'device_del' consecutively. At the end, mirror_run()
> calls to bdrv_drained_begin(), which is scheduled of later (via
> bdrv_co_yield_to_drain()) as the mirror job is running in a coroutine.
> 
> At the same time, the Guest requests the device to be unplugged, which
> leads to blk_unref()->blk_drain()->bdrv_do_drained_begin(). When the
> latter reaches BDRV_POLL_WHILE, the bdrv_drained_begin scheduled above
> is run, which also runs BDRV_POLL_WHILE, leading to the thread getting
> stuck in aio_poll().
> 
> Is it really safe scheduling a bdrv_drained_begin() with poll == true?

I don't see what the problem would be with it in theory. Once the node
becomes idle, both the inner and the outer BDRV_POLL_WHILE() should
return.

The question with such hangs is usually, what is the condition that made
bdrv_drain_poll() return true, and why aren't we making progress so that
is would become false. With iothreads, it could also be that the
condition has actually already changed, but aio_wait_kick() wasn't
called, so aio_poll() isn't woken up.

Kevin