Messages in this thread | | | Date | Sat, 15 Jun 2002 09:55:49 +0200 | From | Jens Axboe <> | Subject | Re: bio_chain: proposed solution for bio_alloc failure and large IO simplification |
| |
On Fri, Jun 14 2002, Adam J. Richter wrote: > On Fri, 14 Jun 2002 Jens Axboe wrote: > >On Thu, Jun 13 2002, Adam J. Richter wrote: > [...] > >> newbio = bio_chain(oldbio); > [...] > >> I realize there may be locking issues in implementing this. > > >but I think that statement is very naive :-) > > >Essentially you are keeping a cookie to the old bio submitted, so that > >you can merge new bio with it fairly cheaply (it's still not very cheap, > >although you do eliminate the biggest offender, the list merge scan). > >The first problem is that once you submit the bio, there are some things > >that are not yours to touch anymore. You cannot change bi_next for > >instance, you have no idea what state the bio is in wrt I/O completion. > >Sure you can hold a reference to the bio, but all that really gets you > >in this situation is a handle to it, nothing more. How do you decide > >when to invalidate this cookie that you have? > > 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. Sure you can grab a reference to the bio with bio_get(), but what would that buy you? Just that the bio at least won't be freed by the time bio_submit returns, but that's it.
-- Jens Axboe
- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
| |