Re: CPU lock-ups with 4.12.0+ kernels related to usb_storage

[Christoph Hellwig <hch@lst.de>]

On Wed, Jul 12, 2017 at 12:10:02PM -0400, Alan Stern wrote:
> This is pretty conclusive.  The problem comes about because
> usb_stor_control_thread() calls scsi_mq_done() while holding
> shost->host_lock, and then scsi_eh_scmd_add() tries to acquire that
> same lock.
> 
> I don't know why this didn't show up in earlier kernels.  I guess some
> element of the call chain listed above must be new in 4.12.
> 
> Christoph, what's the best way to fix this?  Should usb-storage release
> the host lock before issuing the ->scsi_done callback?  If so, does
> that change need to be applied to any kernels before 4.12?

4.12 switched to blk-mq by default, and while the old code used
a softirq for completions, which is always a difference context
the blk-mq code might execute in the same context it's called in.

So yes, for that we'd need to drop host_lock.  But I wonder how
many more of these are lingering somewhere and if we can find
another workaround.