Re: [BUG][bisected 270065e] linux-next fails to boot on powerpc

On 08/18/2017 04:41 PM, Bart Van Assche wrote:

On Fri, 2017-08-18 at 16:04 -0500, Brian King wrote:

quoted

I think I have an understanding what is going on and why Bart's patch is causing problems for ipr.
I can work around the boot hang in ipr, but ultimately I think we need to figure out a fix
in scsi / block. I added some tracing and confirmed its not a matter of commands getting stuck
in ipr. The issue is we are retrying failed commands until we finally run out of time. This is
what I see:

1. sd_revalidate_disk calls scsi_report_opcode
2. ipr RAID arrays don't support MAINTENANCE_IN / MI_REPORT_SUPPORTED_OPERATION_CODES
3. ipr returns the command with DID_ERROR
4. scsi_decide_disposition goes to maybe_retry, increments scmd->retries, and returns NEEDS_RETRY
5. scsi_softirq_done calls scsi_queue_insert to requeue the command, which calls scsi_mq_requeue_cmd
6. With Bart's change, we then clear RQF_DONTPREP in this path, while prior we did not
7. This results in the command getting scmd->retries zeroed out when it gets re-queued,
   since we go through prep again and we lose our retry counter, resulting in lots and lots of retries.
8. Since the default command timeout for an ipr RAID array is 120 seconds, these retries go on for
   quite a long time...
9. Finally, the command has been retried so long we trip over the overall retry timer
   in scsi_softirq_done and we timeout the command.

I'll follow up with a patch to ipr to workaround the hang, but I think we need to somehow preserve
the retry counter in the scsi command, as this will likely cause issues with other drivers. 

Hello Brian,

Thanks for the detailed analysis. This is very helpful. Have you considered
to change the ipr driver such that it terminates REPORT SUPPORTED OPERATION
CODES commands with the appropriate check condition code instead of DID_ERROR?

Yes. That data is actually in the sense buffer, but since I'm also setting DID_ERROR,
scsi_decide_disposition isn't using it. I've got a patch to do just as you suggest,
to stop setting DID_ERROR when there is more detailed error data available, 
but it will need some additional testing before I submit, as it will impact much
more than just this case. 

To add to my analysis above, #9 should not be there... It looks like
jiffies_at_alloc would also be getting reinitialized in this case, resulting in
a perpetual retry, which is what I was seeing.

Thanks,

Brian

-- 
Brian King
Power Linux I/O
IBM Linux Technology Center