linux-kernel - Re: 3.5-rc3: BUG: Dentry still in use (1) [unmount of cgroup cgroup]

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Message-ID: <20120629165809.GB21048@google.com>
Date:	Fri, 29 Jun 2012 09:58:09 -0700
From:	"tj@...nel.org" <tj@...nel.org>
To:	Li Zefan <lizefan@...wei.com>
Cc:	shyju pv <shyju.pv@...wei.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"cgroups@...r.kernel.org" <cgroups@...r.kernel.org>,
	Sanil kumar <sanil.kumar@...wei.com>,
	Masanari Iida <standby24x7@...il.com>
Subject: Re: 3.5-rc3: BUG: Dentry still in use (1) [unmount of cgroup
 cgroup]

Hey,

On Fri, Jun 29, 2012 at 10:20:11AM +0800, Li Zefan wrote:
> > Can you please elaborate a bit?  I'm not really following?  Where does
> > the last root->d_count-- come from?
> 
> 
> From my limited knowledge about vfs internal, seems the parent's refcnt won't go down
> to 0 before its children. When mkdir, the parent's refcnt will be incremented, and
> after rmdir, dput(subdir) will drop subdir's refcnt and then drop parent's.
> 
> So when dropping the subdir's refcnt and leading the superblock to be killed, the root's
> dentry is still > 0.

Heh, yeah, I thought you found who was holding out on the refcnt. :)

> >> I use this script to reproduce the bug:
> >>
> >> mount -t cgroup -o cpu xxx /mnt
> >> mkdir /mnt/sub
> >> sleep 100 < /mnt/sub &
> >> kill $!
> >> wait $!
> >> rmdir /mnt/sub
> >> umount /mnt
> > Unfortunately, this doesn't reproduce the bug here either. :(
> 
> I can reproduce the bug reliably.. Try s/cpu/perf or s/cpu/net_cls, which have fewer
> cgroup files?

Hmm... weird.  Maybe some debug config option I have is preventing the
race from occurring as reliably?  Can you please attach your .config?

Thanks.

-- 
tejun
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