Re: [PATCH] cgroup: superblock can't be released with active dentries

2012/5/24 23:41, Tejun Heo wrote:

quoted

From 88787c483106c5830a46d18deaffdc1e70929af7 Mon Sep 17 00:00:00 2001

From: Tejun Heo <redacted>
Date: Thu, 24 May 2012 08:24:39 -0700

48ddbe1946 "cgroup: make css->refcnt clearing on cgroup removal
optional" allowed a css to linger after the associated cgroup is
removed.  As a css holds a reference on the cgroup's dentry, it means
that cgroup dentries may linger for a while.

cgroup_create() does grab an active reference on the superblock to
prevent it from going away while there are !root cgroups; however, the
reference is put from cgroup_diput() which is invoked on cgroup
removal, so cgroup dentries which are removed but persisting due to
lingering csses already have released their superblock active refs
allowing superblock to be killed while those dentries are around.

Given the right condition, this makes cgroup_kill_sb() call
kill_litter_super() with dentries with non-zero d_count leading to
BUG() in shrink_dcache_for_umount_subtree().

Fix it by adding cgroup_dops->d_release() operation and moving
deactivate_super() to it.  cgroup_diput() now marks dentry->d_fsdata
with itself if superblock should be deactivated and cgroup_d_release()
deactivates the superblock on dentry release.

Signed-off-by: Tejun Heo <redacted>
Reported-by: Sasha Levin <redacted>
Tested-by: Sasha Levin <redacted>
LKML-Reference: [off-list ref]

Acked-by: Li Zefan <redacted>