Re: plug problem in linux-2.4.0-test11

[Jens Axboe <axboe@suse.de>]

On Wed, Nov 29 2000, Linus Torvalds wrote:
> I would much rather actually go back to the original setup, which did
> nothing at all if the queue wasn't plugged in the first place.

But that potentially breaks devices that either don't use plugging
or alternatively implement their own, because q->plugged will not
get set and the unplug from __get_request_wait does nothing. It's
clear that the plug/noplug is too coarse. Anyway, I don't think
there's currently a problem with the old setup for any in-kernel
drivers so I'm fine with reverting to that behaviour for now.

> I think that we should strive for a setup that calls "request_fn" only to
> start new IO, and that expects the low-level driver to be able to do the
> whole request queue until it is empty. Then we re-start it the next time
> around.

Yes agreed.

--- drivers/block/ll_rw_blk.c~	Wed Nov 29 15:17:33 2000
+++ drivers/block/ll_rw_blk.c	Wed Nov 29 19:04:50 2000
@@ -347,9 +347,10 @@
  */
 static inline void __generic_unplug_device(request_queue_t *q)
 {
-	if (!list_empty(&q->queue_head)) {
+	if (q->plugged) {
 		q->plugged = 0;
-		q->request_fn(q);
+		if (!list_empty(&q->queue_head))
+			q->request_fn(q);
 	}
 }
 
-- 
* Jens Axboe <axboe@suse.de>
* SuSE Labs
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