cgroup: superblock can't be released with active dentries (fa980ca8) · Commits · 戴 / test

"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 <tj@kernel.org> Reported-by:

Sasha Levin <levinsasha928@gmail.com> Tested-by:

Sasha Levin <levinsasha928@gmail.com> LKML-Reference: <CA+1xoqe5hMuxzCRhMy7J0XchDk2ZnuxOHJKikROk1-ReAzcT6g@mail.gmail.com> Acked-by:

Li Zefan <lizefan@huawei.com>

kernel/cgroup.c

+14 −3

Original line number	Original line	Diff line number	Diff line
	@@ -896,10 +896,13 @@ static void cgroup_diput(struct dentry dentry, struct inode inode)
	mutex_unlock(&cgroup_mutex);		mutex_unlock(&cgroup_mutex);

	/*		/*
	* Drop the active superblock reference that we took when we		* We want to drop the active superblock reference from the
	* created the cgroup		* cgroup creation after all the dentry refs are gone -
			* kill_sb gets mighty unhappy otherwise. Mark
			* dentry->d_fsdata with cgroup_diput() to tell
			* cgroup_d_release() to call deactivate_super().
	*/		*/
	deactivate_super(cgrp->root->sb);		dentry->d_fsdata = cgroup_diput;

	/*		/*
	* if we're getting rid of the cgroup, refcount should ensure		* if we're getting rid of the cgroup, refcount should ensure
	@@ -925,6 +928,13 @@ static int cgroup_delete(const struct dentry *d)
	return 1;		return 1;
	}		}

			static void cgroup_d_release(struct dentry *dentry)
			{
			/* did cgroup_diput() tell me to deactivate super? */
			if (dentry->d_fsdata == cgroup_diput)
			deactivate_super(dentry->d_sb);
			}

	static void remove_dir(struct dentry *d)		static void remove_dir(struct dentry *d)
	{		{
	struct dentry *parent = dget(d->d_parent);		struct dentry *parent = dget(d->d_parent);
	@@ -1532,6 +1542,7 @@ static int cgroup_get_rootdir(struct super_block *sb)
	static const struct dentry_operations cgroup_dops = {		static const struct dentry_operations cgroup_dops = {
	.d_iput = cgroup_diput,		.d_iput = cgroup_diput,
	.d_delete = cgroup_delete,		.d_delete = cgroup_delete,
			.d_release = cgroup_d_release,
	};		};

	struct inode *inode =		struct inode *inode =

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