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Commit a47295e6 authored by Paul Menage's avatar Paul Menage Committed by Linus Torvalds
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cgroups: make cgroup_path() RCU-safe 



Fix races between /proc/sched_debug by freeing cgroup objects via an RCU
callback.  Thus any cgroup reference obtained from an RCU-safe source will
remain valid during the RCU section.  Since dentries are also RCU-safe,
this allows us to traverse up the tree safely.

Additionally, make cgroup_path() check for a NULL cgrp->dentry to avoid
trying to report a path for a partially-created cgroup.

[lizf@cn.fujitsu.com: call deactive_super() in cgroup_diput()]
Signed-off-by: default avatarPaul Menage <menage@google.com>
Reviewed-by: default avatarLi Zefan <lizf@cn.fujitsu.com>
Tested-by: default avatarLi Zefan <lizf@cn.fujitsu.com>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: default avatarLi Zefan <lizf@cn.fujitsu.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent e7b80bb6

include/linux/cgroup.h

+4 −1
Original line number Diff line number Diff line
@@ -116,7 +116,7 @@ struct cgroup { 
	struct list_head children;	/* my children */



	struct cgroup *parent;	/* my parent */

	struct dentry *dentry;	  	/* cgroup fs entry */

	struct dentry *dentry;	  	/* cgroup fs entry, RCU protected */



	/* Private pointers for each registered subsystem */

	struct cgroup_subsys_state *subsys[CGROUP_SUBSYS_COUNT];
@@ -145,6 +145,9 @@ struct cgroup { 
	int pids_use_count;

	/* Length of the current tasks_pids array */

	int pids_length;



	/* For RCU-protected deletion */

	struct rcu_head rcu_head;

};



/* A css_set is a structure holding pointers to a set of

kernel/cgroup.c

+21 −9
Original line number Diff line number Diff line
@@ -271,7 +271,7 @@ static void __put_css_set(struct css_set *cg, int taskexit) 


	rcu_read_lock();

	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {

		struct cgroup *cgrp = cg->subsys[i]->cgroup;

		struct cgroup *cgrp = rcu_dereference(cg->subsys[i]->cgroup);

		if (atomic_dec_and_test(&cgrp->count) &&

		    notify_on_release(cgrp)) {

			if (taskexit)
@@ -594,6 +594,13 @@ static void cgroup_call_pre_destroy(struct cgroup *cgrp) 
	return;

}



static void free_cgroup_rcu(struct rcu_head *obj)

{

	struct cgroup *cgrp = container_of(obj, struct cgroup, rcu_head);



	kfree(cgrp);

}



static void cgroup_diput(struct dentry *dentry, struct inode *inode)

{

	/* is dentry a directory ? if so, kfree() associated cgroup */
@@ -619,11 +626,13 @@ static void cgroup_diput(struct dentry *dentry, struct inode *inode) 
		cgrp->root->number_of_cgroups--;

		mutex_unlock(&cgroup_mutex);



		/* Drop the active superblock reference that we took when we

		 * created the cgroup */

		/*

		 * Drop the active superblock reference that we took when we

		 * created the cgroup

		 */

		deactivate_super(cgrp->root->sb);



		kfree(cgrp);

		call_rcu(&cgrp->rcu_head, free_cgroup_rcu);

	}

	iput(inode);

}
@@ -1134,14 +1143,16 @@ static inline struct cftype *__d_cft(struct dentry *dentry) 
 * @buf: the buffer to write the path into

 * @buflen: the length of the buffer

 *

 * Called with cgroup_mutex held. Writes path of cgroup into buf.

 * Returns 0 on success, -errno on error.

 * Called with cgroup_mutex held or else with an RCU-protected cgroup

 * reference.  Writes path of cgroup into buf.  Returns 0 on success,

 * -errno on error.

 */

int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen)

{

	char *start;

	struct dentry *dentry = rcu_dereference(cgrp->dentry);



	if (cgrp == dummytop) {

	if (!dentry || cgrp == dummytop) {

		/*

		 * Inactive subsystems have no dentry for their root

		 * cgroup
@@ -1154,13 +1165,14 @@ int cgroup_path(const struct cgroup *cgrp, char *buf, int buflen) 


	*--start = '\0';

	for (;;) {

		int len = cgrp->dentry->d_name.len;

		int len = dentry->d_name.len;

		if ((start -= len) < buf)

			return -ENAMETOOLONG;

		memcpy(start, cgrp->dentry->d_name.name, len);

		cgrp = cgrp->parent;

		if (!cgrp)

			break;

		dentry = rcu_dereference(cgrp->dentry);

		if (!cgrp->parent)

			continue;

		if (--start < buf)
@@ -1663,7 +1675,7 @@ static int cgroup_create_dir(struct cgroup *cgrp, struct dentry *dentry, 
	if (!error) {

		dentry->d_fsdata = cgrp;

		inc_nlink(parent->d_inode);

		cgrp->dentry = dentry;

		rcu_assign_pointer(cgrp->dentry, dentry);

		dget(dentry);

	}

	dput(dentry);