Re: [PATCH 2/5] scsi: make scsi_eh_scmd_add() always succeed

[Christoph Hellwig <hch@lst.de>]

On Thu, Dec 03, 2015 at 08:17:40AM +0100, Hannes Reinecke wrote:
> scsi_eh_scmd_add() currently only will fail if no
> error handler thread is started (which will never be the
> case) or if the state machine encounters an illegal transition.
> 
> But if we're encountering an invalid state transition
> chances is we cannot fixup things with the error handler.
> So better add a WARN_ON for illegal host states and
> make scsi_dh_scmd_add() a void function.

The ehandler parts looks trivially correct, but I'm a little worried
about the state transition.  The states that we can't transition from
are: SHOST_CREATED, SHOST_DEL and SHOST_DEL_RECOVERY.

We initialize the state to SHOST_CREATED in scsi_host_alloc and
transition away from it in scsi_add_host_with_dma, so that's a true
"should be impossible" condition.

We transition to SHOST_DEL or SHOST_DEL_RECOVERY in scsi_remove_host
and the host remains in it until the final reference is dropped. Given
that we wait for all pending I/O in blk_cleanup_queue called from
__scsi_remove_device this should be fine as well.

So:

Reviewed-by: Christoph Hellwig <hch@lst.de>

But preferably with an updated changelog that explains things better.