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

[Christoph Hellwig]

On Mon, Jun 20, 2022 at 06:02:30PM +0900, Damien Le Moal wrote:
> So reserving a tag/req to be able to do NCQ at the cost of max qd being 31
> works for that. We could keep max qd at 32 by creating one more "fake" tag
> and having a request for it, that is, having the fake tag visible to the
> block layer as a reserved tag, as John's series is doing, but for the
> reserved tags, we actually need to use an effective tag (qc->hw_tag) when
> issuing the commands. And for that, we can reuse the tag of one of the
> failed commands.

Take a look at the magic flush request in blk-flush.c, which is
preallocated but borrows a tag from the request that wants a pre- or
post-flush.  The logic is rather ugly, but maybe it might actually
become cleaner by generalizing it a bit.