Hi Martin, Just wanted to check if you planned to apply this v2 (you tried to apply v1 but it didn't compile, so I rebased it onto 5.7/scsi-queue as you requested). Please let me know if there's anything you'd like to see changed. Regards, Merlijn On 18/02/2020 20:21, Merlijn B.W. Wajer wrote: > Hi, > > On 18/02/2020 18:31, Christoph Hellwig wrote: >> On Tue, Feb 18, 2020 at 09:28:34AM -0800, James Bottomley wrote: >>> On Tue, 2020-02-18 at 09:23 -0800, Christoph Hellwig wrote: >>>> On Tue, Feb 18, 2020 at 09:20:28AM -0800, James Bottomley wrote: >>>>>>> Replace the global mutex with per-sr-device mutex. >>>>>> >>>>>> Do we actually need the lock at all? What is protected by it? >>>>> >>>>> We do at least for cdrom_open. It modifies the cdi structure with >>>>> no other protection and concurrent modification would at least >>>>> screw up the use counter which is not atomic. Same reasoning for >>>>> cdrom_release. >>>> >>>> Wouldn't the right fix to add locking to cdrom_open/release instead >>>> of having an undocumented requirement for the callers? >>> >>> Yes ... but that's somewhat of a bigger patch because you now have to >>> reason about the callbacks within cdrom. There's also the question of >>> whether you can assume ops->generic_packet() has its own concurrency >>> protections ... it's certainly true for SCSI, but is it for anything >>> else? Although I suppose you can just not care and run the internal >>> lock over it anyway. >> >> We have 4 instances of struct cdrom_device_ops in the kernel, one of >> which has a no-op generic_packet. So I don't think this should be a >> huge project. > > The are two reasons I decided to make minor changes to fix the > performance regression. > > First, being able to send the patch to the various stable branches once > merged. For people working with many CD drives attached to one station, > this is a pretty big deal, so I tried to keep the patch simple. It fixes > the regression introduced in another commit. > > Secondly, I don't have the hardware to test sophisticated or old setups, > like some of the issues linked from my patch. I have SATA CD drives with > USB->SATA bridges, no IDE, no PATA, etc. So the testing I can do is > relatively limited. > > Perhaps I or someone else can work on removing the usage of the locks, > but as it stands I think this addresses the performance issue present in > the current kernel, and removing locks and the associated testing > required with that is something I am not entirely comfortable doing. > > Cheers, > Merlijn >
