Re: [PATCH RFC v2 03/18] scsi: core: Implement reserved command handling

[John Garry]

On 13/06/2022 10:43, Damien Le Moal wrote:
> > > Currently, that is not possible to do cleanly as there are no guarantees
> > > we can get a free tag (there is a race between block layer tag allocation
> > > and libata internal tag counting). So a reserved tag for that would be
> > > nice. We would end up with 31 IO tags at most + 1 reserved tag for NCQ
> > > commands + ATA_TAG_INTERNAL for non-NCQ. That last one would be rendered
> > > rather useless. But that also means that we kind-of go back to the days
> > > when Linux showed ATA drives max QD of 31...
> > So must the ATA_TAG_INTERNAL qc always be available for non-NCQ action
> > like EH, and that is why you cannot reuse for this internal NCQ
> > (queuable) command?
> Currently, ATA_TAG_INTERNAL is always used for non-NCQ commands. Seeing a
> qc with that tag means it is*not*  NCQ.
>
> I am trying to see if I can reuse the tag from one of the commands that
> completed with that weird good status/sense data available. The problem
> though is that this all needs to be done*before*  calling
> qc->complete_fn() which will free the tag. So we endup with 2 qcs that
> have the same tag, the second one (for the read log command) temporarily
> using the tag but still going through the same completion path without the
> original command fully completed yet. It is a real mess.

Reusing tags seems really messy, but then reserving an NCQ command seems 
wasteful.

Thanks,
John