linux-kernel - Re: kobject lifetime issues in blk-mq

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Date:   Tue, 20 Nov 2018 13:53:47 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Ming Lei <ming.lei@...hat.com>, Guenter Roeck <linux@...ck-us.net>,
        Ming Lei <tom.leiming@...il.com>, Jens Axboe <axboe@...com>,
        Peter Zijlstra <peterz@...radead.org>,
        linux-block@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>,
        Hannes Reinecke <hare@...e.de>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Christoph Hellwig <hch@....de>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        "James E.J. Bottomley" <jejb@...ux.vnet.ibm.com>,
        linux-scsi <linux-scsi@...r.kernel.org>
Subject: Re: kobject lifetime issues in blk-mq

On Tue, Nov 20, 2018 at 1:05 PM, Greg Kroah-Hartman
<gregkh@...uxfoundation.org> wrote:
> 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@...uxfoundation.org> 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 :)


This is great.
What is the patch name? I can't find anything that looks relevant on
LKML searching by kobject.