Messages in this thread | | | Subject | Re: [RFC PATCH] scsi: fix oops in scsi_uninit_cmd() | From | Jason Yan <> | Date | Thu, 14 Mar 2019 09:57:19 +0800 |
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On 2019/3/14 7:51, Bart Van Assche wrote: > On Thu, 2019-02-21 at 16:53 +0800, Jason Yan wrote: >> On 2019/2/20 23:18, Christoph Hellwig wrote: >>> [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? >> >> When userspace close the fd after blk_update_request() and before >> scsi_mq_uninit_cmd(), a disk reference will be released. It is not the >> blk_update_request() directly released it. >> >> close >> ->sd_release >> ->scsi_disk_put >> ->scsi_disk_release >> ->disk->private_data = NULL; >> >> The userspace can close the fd because blk_update_request() returned the >> last IO , the userspace application does not have to stuck on read() or >> write(). The window is very small, but it can be reproduce every day >> in our testcases. So I'm very curious why. One possible explanation is >> that we enabled kernel preempt(CONFIG_PREEMPT). >> >> And why can't we move that release to __blk_mq_end_request? > > Hi Jason, > > What is the current status of this issue? >
Hi Bart,
I did not find any other approach that will not affect the hot path. I don't know if you guys have other suggestions?
> Thanks, > > Bart. > > . >
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