Re: Why we were seeing so many spurious NCQ completions

[Jeff Garzik <jeff@garzik.org>]

Tejun Heo wrote:
> Hello, all.
> 
> This has been going on for quite some time now but I finally succeeded
> to reproduce the problem and find out what has been going on.  It
> wasn't drive's or controller's fault.  The spurious completion
> detection logic was wrong which makes all of this my fault.  :-)
> 
> The attached patch induces NCQ spurious completions by inserting
> artificial delays during irq handling.  The following is log with the
> patch applied.
> 
> A [ 1125.478813] ata35: MON issue=0x0 SAct=0x1 sactive=0x3 SDB FIS=004040a1:00000002
> B [ 1125.480248] ata35: MON issue=0x4 SAct=0x6 sactive=0x7 SDB FIS=004040a1:00000001
> C [ 1125.481614] ata35: MON issue=0x0 SAct=0x5 sactive=0x7 SDB FIS=004040a1:00000002
> D [ 1125.481704] ata35: YYY 0x2 -> 0x4
> E [ 1125.481722] ata35: XXX issue=0x0 SAct=0x1 sactive=0x1 SDB FIS=004040a1:00000004
> F [ 1125.483087] ata35: MON issue=0x0 SAct=0x0 sactive=0x1 SDB FIS=004040a1:00000001
> G [ 1125.484297] ata35: MON issue=0x4 SAct=0x6 sactive=0x7 SDB FIS=004040a1:00000001

Thanks a lot for tracking this down, and thanks even more for being 
humble enough to admit mistakes.  More kernel hackers should follow your 
example.

I continue to be a proud mentor, watching you kick ass on the Linux 
kernel scene :)

	Jeff