All of lore.kernel.org
 help / color / mirror / Atom feed
From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Tejun Heo <tj@kernel.org>
Cc: Yufen Yu <yuyufen@huawei.com>,
	axboe@kernel.dk, linux-block@vger.kernel.org,
	linux-fsdevel@vger.kernel.org, jack@suse.cz, bvanassche@acm.org,
	tytso@mit.edu
Subject: Re: [PATCH v2 3/7] bdi: protect device lifetime with RCU
Date: Fri, 6 Mar 2020 17:25:47 +0100	[thread overview]
Message-ID: <20200306162547.GB3838587@kroah.com> (raw)
In-Reply-To: <20200305012211.GA33199@mtj.duckdns.org>

On Wed, Mar 04, 2020 at 08:22:11PM -0500, Tejun Heo wrote:
> Hello,
> 
> On Wed, Mar 04, 2020 at 09:05:59PM +0100, Greg Kroah-Hartman wrote:
> > > Lifetime rules in block layer are kinda nebulous. Some of it comes
> > > from the fact that some objects are reused. Instead of the usual,
> > > create-use-release, they get repurposed to be associated with
> > > something else. When looking at such an object from some paths, we
> > > don't necessarily have ownership of all of the members.
> > 
> > That's horrid, it's not like block devices are on some "fast path" for
> > tear-down, we should do it correctly.
> 
> Yeah, it got retrofitted umpteenth times from the really early days. I
> don't think much of it is intentionally designed to be this way.
> 
> > > > backing_device_info?  Are these being destroyed/used so often that rcu
> > > > really is the best solution and the existing reference counting doesn't
> > > > work properly?
> > > 
> > > It's more that there are entry points which can only ensure that just
> > > the top level object is valid and the member objects might be going or
> > > coming as we're looking at it.
> > 
> > That's not ok, a "member object" can only be valid if you have a
> > reference to it.  If you remove the object, you then drop the reference,
> > shouldn't that be the correct thing to do?
> 
> I mean, it depends. There are two layers of objects and the top level
> object has two stacked lifetime rules. The "active" usage pins
> everything as usual. The "shallower" usage only has full access to the
> top level and when it reaches down into members it needs a different
> mechanism to ensure its validity. Given a clean slate, I don't think
> we'd go for this design for these objects but the usage isn't
> fundamentally broken.
> 
> Idk, for the problem at hand, the choice is between patching it up by
> copying the name and RCU protecting ->dev access at least for now.
> Both are nasty in their own ways but copying does have a smaller blast
> radius. So, copy for now?

Yes, copy for now, don't mess with RCU and the struct device lifetime,
that is not going to solve anything.

I'll put the "fix the lifetime rules in the block layer" on my todo
list, at the bottom :(

thanks,

greg k-h

  reply	other threads:[~2020-03-06 16:25 UTC|newest]

Thread overview: 22+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-02-26 11:18 [PATCH v2 0/7] bdi: fix use-after-free for bdi device Yufen Yu
2020-02-26 11:18 ` [PATCH v2 1/7] blk-wbt: use bdi_dev_name() to get device name Yufen Yu
2020-02-26 11:18 ` [PATCH v2 2/7] fs/ceph: " Yufen Yu
2020-02-26 11:18 ` [PATCH v2 3/7] bdi: protect device lifetime with RCU Yufen Yu
2020-03-04 17:05   ` Tejun Heo
2020-03-04 17:22     ` Greg Kroah-Hartman
2020-03-04 17:23       ` Greg Kroah-Hartman
2020-03-04 18:50       ` Tejun Heo
2020-03-04 19:10         ` Theodore Y. Ts'o
2020-03-04 19:15           ` Tejun Heo
2020-03-04 20:05         ` Greg Kroah-Hartman
2020-03-05  1:22           ` Tejun Heo
2020-03-06 16:25             ` Greg Kroah-Hartman [this message]
2020-03-07  9:13               ` Yufen Yu
2020-02-26 11:18 ` [PATCH v2 4/7] bdi: create a new function bdi_get_dev_name() Yufen Yu
2020-02-26 11:18 ` [PATCH v2 5/7] bfq: fix potential kernel crash when print dev err info Yufen Yu
2020-02-26 11:18 ` [PATCH v2 6/7] memcg: fix crash in wb_workfn when bdi unregister Yufen Yu
2020-02-26 11:18 ` [PATCH v2 7/7] blk-wbt: replace bdi_dev_name() with bdi_get_dev_name() Yufen Yu
2020-03-04 17:29 ` [PATCH v2 0/7] bdi: fix use-after-free for bdi device Greg KH
2020-03-04 18:57   ` Tejun Heo
2020-03-04 20:07     ` Greg KH
2020-03-04 19:02   ` Theodore Y. Ts'o

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20200306162547.GB3838587@kroah.com \
    --to=gregkh@linuxfoundation.org \
    --cc=axboe@kernel.dk \
    --cc=bvanassche@acm.org \
    --cc=jack@suse.cz \
    --cc=linux-block@vger.kernel.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=tj@kernel.org \
    --cc=tytso@mit.edu \
    --cc=yuyufen@huawei.com \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.