Messages in this thread | ![/](/images/icornerl.gif) | | Date | Wed, 14 May 2003 10:32:24 +0200 | From | Jens Axboe <> | Subject | Re: 2.5.69-mjb1: undefined reference to `blk_queue_empty' |
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On Wed, May 14 2003, Bharata B Rao wrote: > On Tue, May 13, 2003 at 08:11:55PM +0200, Jens Axboe wrote: > > > > > > > > /* For now we assume we have the device to ourselves */ > > > > /* Just a quick sanity check */ > > > > - if (!blk_queue_empty(bdev_get_queue(dump_bdev->bdev))) { > > > > + if (elv_next_request(bdev_get_queue(dump_bdev->bdev))) { > > > > /* i/o in flight - safer to quit */ > > > > return -EBUSY; > > > > } > > > > this looks horribly racy (of the io scheduler internals corrupting > > kind), I don't see you holding the queue lock here. some io schedulers > > do non-significant amount of work inside they next_request functions, > > moving from back-end lists to dispatch queue. > > > > Jens, > > All we want to do here is to check if there are requests in the > queue. Hence thinking of using elv_queue_empty(). Do you think > we still need to acquire queue lock for this ? This code will be > run when we have stopped everything else in other cpus by putting > them into spin.
That really has to be locked down as well. For your purpose, I think the use of elv_queue_empty() is much better even though it really is an internal function. The problem mainly comes from AS, that can have non empty queue but still return NULL in elv_next_request().
But yes, it needs to be locked. If you have pinned the other CPUs, then I suppose it should work. But it's still a violation of the locking rules, and one would get in trouble dropping the queue lock from the io scheduler elevator_queue_empty_fn. No one does that currently, but... So please take the lock.
-- Jens Axboe
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