[dm-devel] Re: [PATCH][RFC] Use bio markers for request callback

[Jens Axboe]

On Tue, Sep 11 2007, Hannes Reinecke wrote:
> Hi all,
>
> this is a proposal for a different implementation of request callbacks. The 
> existing ->endio callback of a request is actually a destructor function, 
> to be called to terminate a request and free all structures.
>
> However, on certain occasions (like request-based multipathing) it is 
> desirable to have a callback function for a request which is called right 
> after the request is finished, ie in end_that_request_first() before any 
> bio->bi_endio callback is called.
>
> So a simple solution for this is to clone the request and add a new 
> 'marker' bio in front of the bio list of the request. This callback will be 
> attached a structure in bi_private which keeps a pointer to the cloned and 
> the original request, thus serving as a callback for the request itself.
>
> Proposed patch attached. As usual comments are welcome.

Honestly, I think this approach is a much worse design than the NEC
callback option. Exporting __make_request() (which is an INTERNAL
function) aside, this looks like one big hack with pseudo bio attached
to front of list, the enable/disable elevator stuff (does that even work
on stacked devices?).

-- 
Jens Axboe