Re: [PATCH] block: eliminate ELEVATOR_INSERT_REQUEUE (was: Re:elevator private data for REQ_FLUSH)

[Tejun Heo]

On Wed, Mar 30, 2011 at 09:41:23AM +0200, Tejun Heo wrote:
> IIRC, at least in the requeue path, some drivers depend on front
> queueing for forward progress guarantee.  Forward progress for
> prepping is guaranteed by mempool (or something like that) and when
> the request is retried, it should stay at the front of the queue;
> otherwise, prepping can stall with prepped requests stuck behind
> unprepped ones.

After writing the above, I think the current flush implementation
actually is violating the above.  I think the problem is over-use of
front insertion.  Flush can just append to the dispatch queue.  Front
insertion should only be used by requeueing, really.

Thanks.

-- 
tejun
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