Re: [PATCH] 2.5.15 IDE 61

[Neil Conway <nconway.list@ukaea.org.uk>]

Jens Axboe wrote:
> On Tue, May 14 2002, Neil Conway wrote:
> > that's all the actual spinlock buys you.  It does not IIUC mean that you
> > can't get a call to request_fn of one queue while the other queue has
> > lots of requests in it (which are potentially being serviced by DMA).
> 
> Bingo, this is exactly right and makes the point a hell of a lot better
> than I did in my previous mail. Shared locks will only buy you that
> noone fiddles with one list while the other is busy (ie nothing for us).

Cool, thanks ;-)  Now watch me blow all my cred with this post:

> To really serialize operations the queue _must_ be shared with whoever
> requires serialiation.

Why will this help?  The hardware can still be doing DMA on hda while
the queue's request_fn is called quite legitimately for a hdb request -
and the IDE code MUST impose the serialization here to avoid hitting the
cable with commands destined for hdb. (For example, by waiting for
!channel->busy.)

> If not, the problem will have to be solved at the IDE level, not the
> block level. And that has not looked pretty in the past.

I just can't see a way for the block level to remove the need for the
busy flag.  I _think_ Alan just agreed with me.  I'm not sure but I get
the impression that you are saying the IDE code doesn't need to do this
serialization...

I'm certainly learning, thanks guys.
Neil