Messages in this thread | | | From | Bartlomiej Zolnierkiewicz <> | Subject | Re: Possibly wrong BIO usage in ide_multwrite | Date | Fri, 2 Jan 2004 01:27:50 +1400 |
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On Friday 02 January 2004 07:18, Christophe Saout wrote: > Hi!
Hi,
> I was just investigating where bio->bi_idx gets modified in the kernel. > > I found these lines in ide-disk.c in ide_multwrite (DMA off, TASKFILE_IO > > off): > > if (++bio->bi_idx >= bio->bi_vcnt) { > > bio->bi_idx = 0; > > bio = bio->bi_next; > > } > > (rq->bio also gets changed but it's protected by the scratch buffer) > > I think changing the bi_idx here is dangerous because > end_that_request_first needs this value to be unchanged because it > tracks the progress of the bio processing and updates bi_idx itself.
This is not a problem here because ide_multwrite() walks rq->bio chain itself. It also updates current_nr_sectors and hard_cur_sectors fields of drive->wrq.
> And bio->bi_idx = 0 is probably wrong because the bio can be submitted > with bio->bi_idx > 0 (if the bio was splitted and there are clones that > share the bio_vec array, like raid or device-mapper code). > > If it really needs to play with bi_idx itself care should be taken to > reset bi_idx to the original value, not to zero.
RAID or device-mapper code doesn't seem to care about bio->bi_idx value after bio has been submitted to the block layer, so the current code is safe enough. Also there is no place to store original bi_idx.
> I wasn't able to trigger a problem though, I don't know why exactly, > perhaps there are paths in __end_that_request_first that are not > interested in bi_dx. I still think there is something wrong with it.
After finishing data transfer multwrite_intr() calls ide_end_request() with rq->nr_sectors argument (where rq is hwgroup->rq not drive->wrq), so only whole bios are completed. There are no partial completions and code depending on bio->bi_idx inside __end_that_request_first() is not executed.
The real (generic) problem is that atomic block segment for a block device (in this case ATA disk) can be composed of bvecs, bios or bios+bvecs and driver can obtain information about next bvec from block layer (from rq->bio) only after previous bvec has been acknowledgment by end_that_request_first(). In situation when information about previously processed bios/bvecs is needed (ie. error condition) this information is already lost.
There are 2 solutions for this problem:
- Use separate bio lists (rq->cbio) and temporary data (rq->nr_cbio_segments and rq->nr_cbio_sectors) for submission/completion. Please look at process_that_request_first() and its usage in TASKFILE code.
You are then required to do partial bio completion.
- Do not use struct request fields directly and store information needed by driver in separate data structures (ie. scatter-gather data stored by SCSI layer).
You are then not allowed (and shouldn't need) to do partial bio completions.
IDE multi-sector code uses first method because second one was too invasive/risky to be done (required serious rewrite of IDE transport code).
I hope generic block transport layer in 2.7.x will make life simpler.
--bart
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