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From: David Howells <dhowells@redhat.com>
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References: <20190102233118.GA19202@gmail.com> <20181229000400.26333-1-avagin@gmail.com> <20181228235900.21468-1-avagin@gmail.com> <21902.1546467980@warthog.procyon.org.uk> <20190102230655.GA8616@gmail.com>
To: Andrei Vagin <avagin@gmail.com>
Cc: dhowells@redhat.com, Alexander Viro <viro@zeniv.linux.org.uk>,
        linux-fsdevel@vger.kernel.org, cgroups@vger.kernel.org,
        Li Zefan <lizefan@huawei.com>
Subject: Re: [PATCH vfs/for-next v2] cgroup: fix top cgroup refcnt leak
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Date: Thu, 03 Jan 2019 00:33:13 +0000
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Andrei Vagin <avagin@gmail.com> wrote:

> Here is a reproducer for this problem:
> 
> [root@fc24 ~]# cat fs-vs-cg 
> set -m
> d=$(mktemp -d /tmp/cg.XXXXXX)
> for i in `seq 2`; do 
> 	mkdir $d/a$i
> 	mount -t cgroup -o none,name=xxxxy xxx $d/a$i
> done
> mkdir -p $d/a1/test
> for i in `seq 2`; do 
> 	umount $d/a$i
> done
> d=$(mktemp -d /tmp/cg.XXXXXX)
> for i in `seq 2`; do 
> 	mkdir $d/a$i
> 	mount -t cgroup -o none,name=xxxxy xxx $d/a$i
> done
> rmdir $d/a1/test
> for i in `seq 2`; do 
> 	umount $d/a$i
> done
> 
> You need to execute this script twice and it will stuck on a second
> attmept.

This causes:

	percpu ref (css_release) <= 0 (0) after switching to atomic

without your patch and:

	percpu ref (css_release) <= 0 (-2) after switching to atomic

with your patch, both followed by:

	WARNING: CPU: 0 PID: 0 at lib/percpu-refcount.c:155 percpu_ref_switch_to_atomic_rcu+0x90/0x1a0

on Al's for-next branch.

David