Hi, Jens,
Thanks for responding to my mail.
> > - Once the lock has been dropped, it would seem that nothing prevents
> > the system from calling _request() again, should more requests be added
> > to the queue - perhaps even when the first invocation is still running.
> > Correct?
>
> The request fn will not be reinvoked until the queue has been plugged
> again, and that requires that the queue has been emptied. In 2.4
> devices use plugging by default.
I think that is not true. __make_request will call the request function
anytime a request is added and the queue is not plugged. Since plugging
only happens when the queue is empty, a block driver with requests sitting
on the queue can, in fact, see multiple calls to the request function. I
was able to demonstrate this to myself in a test driver by introducing a
suitably long delay before actually removing requests from the queue.
> If you exit before having processed all requests in
> the queue, you need to replug -- but I would consider doing that from
> the driver pretty hacky.
Really? From ll_rw_blk.c:
> @rfn is not required, or even expected, to remove all requests off the
> queue, but only as many as it can handle at a time. If it does leave
> requests on the queue, it is responsible for arranging that the requests
> get dealt with eventually.
>From that I had concluded that leaving requests on the queue was cool - as
long as they don't stay there forever. A number of drivers seem to work
that way - the floppy driver, for example, doesn't bother to dequeue
requests until they are done.
> You can get away with not holding io_request_lock when calling
> end_request (or end_that_request_*) if it is the first request on the
> queue.
One may be able to "get away" with it now - but should I advise people to
do things that way in the driver book? My inclination is to tell people to
be a bit more careful.
Thanks muhc for your help on this matter... with luck I'll actually get
all this stuff straight before long.
jon
Jonathan Corbet
Executive editor, LWN.net
corbet@eklektix.com
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This archive was generated by hypermail 2b29 : Tue Aug 15 2000 - 21:00:15 EST