On Tue, Nov 20, 2018 at 12:34:40PM +0100, Dmitry Vyukov wrote: > On Thu, Nov 15, 2018 at 1:56 AM, Greg Kroah-Hartman > <gregkh@xxxxxxxxxxxxxxxxxxx> wrote: > > On Thu, Nov 15, 2018 at 08:36:17AM +0800, Ming Lei wrote: > >> > So even if you think the kernel is not going to do this, remember, you > >> > have no control over it. Reference counted objects are done this way > >> > for a reason, you really do not know who has a reference and you really > >> > do not care. > >> > > >> > You are just papering over the real issue here, see my previous email > >> > for how to start working on resolving it. > >> > >> IMO, there isn't real issue, and the issue is actually in 'delay release'. > > > > Nope, sorry, that is not true. > > > >> Please look at the code in block/blk-mq-sysfs.c, both q->mq_kobj and all > >> ctx->kobj share same lifetime with q->kobj, we even don't call get/put > >> on q->mq_kobj & all ctx->kobj, and all are simply released in q->kobj's > >> release handler. > > > > How do you "know" you are keeping those lifetimes in sync? The joy of a > > kobject is that _ANYTHING_ can grab a reference to your object without > > you knowing about it. That includes userspace programs. Yes, sysfs is > > now much better and it trys to release that reference "quickly" when it > > determines you are trying to delete a kobject, but it's not perfict, > > there are still races there. > > > > And that is what the delay release code is showing you. It is showing > > you that you "think" your reference counting is wrong, but it is not. > > It is showing you that if someone else grabs a reference, you are not > > correctly cleaning up for yourself. > > > > Never think that you really know the lifetime of a kobject, once you > > realize that your code gets simpler and you can then just "trust" that > > the kernel will do the right thing no matter what. > > > > Because really, you are using a kobject because you want that correct > > reference counting logic. By ignoring that logic, you are ignoring the > > reason to be using that object at all. If you don't need reference > > counting, then don't use it at all. > > > > And if you need sysfs files, then you need to use the kobject and then > > you need to handle it properly, because again, you do NOT have full > > control over the lifetime of your object. That's the basis for > > reference counting in the firstplace. > > > > So this code is broken without me evening having to look at it, please > > fix it to handle release properly. Again, the kernel tried to tell you > > this, but you hacked around the kernel core to remove that warning > > incorrectly. Please go read the kobject documentation again for even > > more details about this than what I said here. > > > > thanks, > > > > greg k-h > > Whoever is the right person to fix this, please prioritize this to the > degree possible. > This issue does not allow to use DEBUG_KOBJECT_RELEASE in any > automated testing (in particular syzbot) on both upstream and stable > trees. We have to disable it for now, so other bugs won't be noticed > and will pile up. Patches for this have already been posted :)
