Re: [RFC PATCH] scsi: fix oops in scsi_uninit_cmd()

[Christoph Hellwig]

[fullquote removed, please follow proper mail etiquette]

On Tue, Feb 19, 2019 at 08:56:28AM -0800, Bart Van Assche wrote:
> regression in the SCSI sd driver due to the switch from the legacy block
> layer to scsi-mq. The above patch introduces two atomic operations in the
> hot path and hence would introduce a performance regression. I think this
> can be avoided by making sure that sd_uninit_command() gets called before
> the request tag is freed. What changes would be required to make the block
> layer core call sd_uninit_command() before the request tag is freed? Would
> introducing prep_rq_fn and unprep_rq_fn callbacks in struct blk_mq_ops and
> making sure that the SCSI core sets these callback function pointers
> appropriately be sufficient? Would such a change allow to simplify the NVMe
> initiator driver? Are there any alternatives to this approach that are more
> elegant?

Additional indirect calls in the I/O fast path is something I'd rather
avoid.  But I don't fully understand the problem yet - where do
we release a disk reference from blk_update_request?  And why can't
we move that release to __blk_mq_end_request?

> 
> Thanks,
> 
> Bart.
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