Re: [PATCH 1/1] block: rework flush sequencing for blk-mq

[Shaohua Li <shli@kernel.org>]

On Thu, Jan 30, 2014 at 05:26:30AM -0800, Christoph Hellwig wrote:
> Witch to using a preallocated flush_rq for blk-mq similar to what's done
> with the old request path.  This allows us to set up the request properly
> with a tag from the actually allowed range and ->rq_disk as needed by
> some drivers.  To make life easier we also switch to dynamic allocation
> of ->flush_rq for the old path.
> 
> This effectively reverts most of
> 
>     "blk-mq: fix for flush deadlock"
> 
> and
> 
>     "blk-mq: Don't reserve a tag for flush request"

Reusing the tag for flush request is considered before. The problem is driver
need get a request from a tag, reusing tag breaks this. The possible solution
is we provide a blk_mq_tag_to_request, and force driver uses it. And in this
function, if tag equals to flush_rq tag, we return flush_request.

Thanks,
Shaohua