From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Dmitry Vyukov <dvyukov@google.com>
Cc: Ming Lei <ming.lei@redhat.com>,
Guenter Roeck <linux@roeck-us.net>,
Ming Lei <tom.leiming@gmail.com>, Jens Axboe <axboe@fb.com>,
Peter Zijlstra <peterz@infradead.org>,
linux-block@vger.kernel.org, LKML <linux-kernel@vger.kernel.org>,
Hannes Reinecke <hare@suse.de>,
Paolo Bonzini <pbonzini@redhat.com>,
Christoph Hellwig <hch@lst.de>,
"Martin K. Petersen" <martin.petersen@oracle.com>,
"James E.J. Bottomley" <jejb@linux.vnet.ibm.com>,
linux-scsi <linux-scsi@vger.kernel.org>
Subject: Re: kobject lifetime issues in blk-mq
Date: Tue, 20 Nov 2018 13:05:40 +0100 [thread overview]
Message-ID: <20181120120540.GA14861@kroah.com> (raw)
In-Reply-To: <CACT4Y+bSzPM022bGMmj4GQ_YHVsVqd6r+NTC4+fRrRT+yjAjEw@mail.gmail.com>
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@linuxfoundation.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 :)
next prev parent reply other threads:[~2018-11-20 12:05 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-11-09 20:35 kobject lifetime issues in blk-mq Guenter Roeck
2018-11-12 9:20 ` Ming Lei
2018-11-12 9:44 ` Ming Lei
2018-11-12 16:48 ` Guenter Roeck
2018-11-12 20:02 ` Greg Kroah-Hartman
2018-11-13 0:22 ` Ming Lei
2018-11-13 0:41 ` Ming Lei
2018-11-14 11:08 ` Ming Lei
2018-11-14 15:14 ` Greg Kroah-Hartman
2018-11-15 0:36 ` Ming Lei
2018-11-15 0:56 ` Greg Kroah-Hartman
2018-11-20 11:34 ` Dmitry Vyukov
2018-11-20 12:05 ` Greg Kroah-Hartman [this message]
2018-11-20 12:53 ` Dmitry Vyukov
2018-11-20 13:27 ` Greg Kroah-Hartman
2018-11-20 13:28 ` Ming Lei
2018-11-14 15:12 ` Greg Kroah-Hartman
2018-11-12 16:16 ` Guenter Roeck
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