Messages in this thread | | | Date | Sat, 15 Jun 2002 10:50:11 +0200 | From | Jens Axboe <> | Subject | Re: bio_chain: proposed solution for bio_alloc failure and large IO simplification |
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On Sat, Jun 15 2002, Adam J. Richter wrote: > >> At any time, there could be only one "hinted" bio in a > >> request: the last bio in the request. So you only have to > >> clear the hint when: > >> > >> 1. you merge bio's, > >> 2. elv_next_request is called, > >> 3. newbio is submitted. > >> > >> In all three cases q->queue_lock gets taken, so we should > >> not need to add any additional spin_lock_irq's, and the two lines > >> to clear the hint pointers should be trivial. > > >This logic is flawed. As I said, once you pass the bio to submit_bio, > >you can't maintain a pointer to it for these purposes. Grabbing the > >queue_lock guarentees absolutely nothing in this regard. Consider loop, > >for instance. I/O could be completed by the time bio_submit returns. > > So, I need a fourth location at in generic_make_request > just before the call to q->make_request_fn, like so: > > if (q->make_request_fn != __make_request) { > int flags; > spin_lock_irqsave(q->lock, flags); > clear_hint(bio); > spin_unlock_irqrestore(q->lock, flags); > } > ret = q->make_request_fn(q, bio);
Irk, this is ugly. But how you are moving away from the initial goal (or maybe this was your goal the whole time, just a single merge hint?) of passing back the hint instead of maintaing it in the queue. So let me ask, are you aware of the last_merge I/O scheduler hint? Which does exactly this already...
-- Jens Axboe
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