Re: [RFC PATCH 0/4] nvme-tcp: fix hung issues for deleting

[许春光]

Hi Ming:

Ming Lei <ming.lei@xxxxxxxxxx> 于2023年6月6日周二 23:15写道：
>
> Hello Chunguang,
>
> On Mon, May 29, 2023 at 06:59:22PM +0800, brookxu.cn wrote:
> > From: Chunguang Xu <chunguang.xu@xxxxxxxxxx>
> >
> > We found that nvme_remove_namespaces() may hang in flush_work(&ctrl->scan_work)
> > while removing ctrl. The root cause may due to the state of ctrl changed to
> > NVME_CTRL_DELETING while removing ctrl , which intterupt nvme_tcp_error_recovery_work()/
> > nvme_reset_ctrl_work()/nvme_tcp_reconnect_or_remove().  At this time, ctrl is
>
> I didn't dig into ctrl state check in these error handler yet, but error
> handling is supposed to provide forward progress for any controller state.
>
> Can you explain a bit how switching to DELETING interrupts the above
> error handling and breaks the forward progress guarantee?

Here we freezed ctrl, if ctrl state has changed to DELETING or
DELETING_NIO(by nvme disconnect),  we will break up and lease ctrl
freeze, so nvme_remove_namespaces() hang.

static void nvme_tcp_error_recovery_work(struct work_struct *work)
{
        ...
        if (!nvme_change_ctrl_state(ctrl, NVME_CTRL_CONNECTING)) {
                /* state change failure is ok if we started ctrl delete */
                WARN_ON_ONCE(ctrl->state != NVME_CTRL_DELETING &&
                             ctrl->state != NVME_CTRL_DELETING_NOIO);
                return;
        }

        nvme_tcp_reconnect_or_remove(ctrl);
}


Another path, we will check ctrl state while reconnecting, if it changes to
DELETING or DELETING_NIO, we will break up and lease ctrl freeze and
queue quiescing (through reset path), as a result Hang occurs.

static void nvme_tcp_reconnect_or_remove(struct nvme_ctrl *ctrl)
{
        /* If we are resetting/deleting then do nothing */
        if (ctrl->state != NVME_CTRL_CONNECTING) {
                WARN_ON_ONCE(ctrl->state == NVME_CTRL_NEW ||
                        ctrl->state == NVME_CTRL_LIVE);
                return;
        }
        ...
}

> > freezed and queue is quiescing . Since scan_work may continue to issue IOs to
> > load partition table, make it blocked, and lead to nvme_tcp_error_recovery_work()
> > hang in flush_work(&ctrl->scan_work).
> >
> > After analyzation, we found that there are mainly two case:
> > 1. Since ctrl is freeze, scan_work hang in __bio_queue_enter() while it issue
> >    new IO to load partition table.
>
> Yeah, nvme freeze usage is fragile, and I suggested to move
> nvme_start_freeze() from nvme_tcp_teardown_io_queues to
> nvme_tcp_configure_io_queues(), such as the posted change on rdma:
>
> https://lore.kernel.org/linux-block/CAHj4cs-4gQHnp5aiekvJmb6o8qAcb6nLV61uOGFiisCzM49_dg@xxxxxxxxxxxxxx/T/#ma0d6bbfaa0c8c1be79738ff86a2fdcf7582e06b0

While drive reconnecting, I think we should freeze ctrl or quiescing queue,
otherwise nvme_fail_nonready_command()may return BLK_STS_RESOURCE,
and the IOs may retry frequently. So I think we may better freeze ctrl
while entering
error_recovery/reconnect, but need to unfreeze it while exit.


> > 2. Since queus is quiescing, requeue timeouted IO may hang in hctx->dispatch
> >    queue, leading scan_work waiting for IO completion.
>
> That still looks one problem in related error handling code, which is
> supposed to recover and unquiesce queue finally.

If I have not misunderstood that is what this patchset does.

Thanks.
>
> Thanks,
> Ming
>