Re: lockdep WARNING at blktests block/011

[Shinichiro Kawasaki <shinichiro.kawasaki@wdc.com>]

On Oct 03, 2022 / 09:28, Keith Busch wrote:
> On Mon, Oct 03, 2022 at 01:32:41PM +0000, Shinichiro Kawasaki wrote:
> > 
> > BTW, I came up to another question during code read. I found nvme_reset_work()
> > calls nvme_dev_disable() before nvme_sync_queues(). So, I think the NVME
> > controller is already disabled when the reset work calls nvme_sync_queues().
> 
> Right, everything previously outstanding has been reclaimed, and the queues are
> quiesced at this point. There's nothing for timeout work to wait for, and the
> sync is just ensuring every timeout work has returned.

Thank you Keith, good to know that we do not need to worry about the deadlock
scenario with nvme_reset_work() :)  I also checked nvme_reset_prepare() and
nvme_suspend(). They also call nvme_dev_disable() or nvme_start/wait_freeze()
before nvme_sync_queues(). So I think the queues are quiesced in these context
also, and do not worry them either.

> 
> It looks like a timeout is required in order to hit this reported deadlock, but
> the driver ensures there's nothing to timeout prior to syncing the queues. I
> don't think lockdep could reasonably know that, though.

I see... From blktests point of view, I would like to suppress the lockdep splat
which triggers the block/011 failure. I created a quick patch using
lockdep_off() and lockdep_on() which skips lockdep check for the read lock in
nvme_sync_io_queues() [1] and confirmed it avoids the lockdep splat. I think
this lockdep check relax is fine because nvme_sync_io_queues() callers do call
nvme_dev_disable(), nvme_start/wait_freeze() or nvme_stop_queues(). The queues
are quiesced at nvme_sync_io_queues() and the deadlock scenario does not happen.

Any comment on this patch will be appreciated. If this action approach is ok,
I'll post as a formal patch for review.

[1]

diff --git a/drivers/nvme/host/core.c b/drivers/nvme/host/core.c
index 66446f1e06c..9d091327a88 100644
--- a/drivers/nvme/host/core.c
+++ b/drivers/nvme/host/core.c
@@ -5121,10 +5121,14 @@ void nvme_sync_io_queues(struct nvme_ctrl *ctrl)
 {
        struct nvme_ns *ns;

+       lockdep_off();
        down_read(&ctrl->namespaces_rwsem);
+       lockdep_on();
        list_for_each_entry(ns, &ctrl->namespaces, list)
                blk_sync_queue(ns->queue);
+       lockdep_off();
        up_read(&ctrl->namespaces_rwsem);
+       lockdep_on();
 }
 EXPORT_SYMBOL_GPL(nvme_sync_io_queues);

-- 
Shin'ichiro Kawasaki