Message-Id: <20090123150227.9ec4360f.nishimura@mxp.nes.nec.co.jp>
Date:	Fri, 23 Jan 2009 15:02:27 +0900
From:	Daisuke Nishimura <nishimura@....nes.nec.co.jp>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	nishimura@....nes.nec.co.jp,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"menage@...gle.com" <menage@...gle.com>,
	"lizf@...fujitsu.com" <lizf@...fujitsu.com>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/7] cgroup: add CSS ID

> +/**
> + * css_get_next - lookup next cgroup under specified hierarchy.
> + * @ss: pointer to subsystem
> + * @id: current position of iteration.
> + * @root: pointer to css. search tree under this.
> + * @foundid: position of found object.
> + *
> + * Search next css under the specified hierarchy of rootid. Calling under
> + * rcu_read_lock() is necessary. Returns NULL if it reaches the end.
> + */
> +struct cgroup_subsys_state *
> +css_get_next(struct cgroup_subsys *ss, int id,
> +	     struct cgroup_subsys_state *root, int *foundid)
> +{
> +	struct cgroup_subsys_state *ret = NULL;
> +	struct css_id *tmp;
> +	int tmpid;
> +	int rootid = css_id(root);
> +	int depth = css_depth(root);
> +
> +	if (!rootid)
> +		return NULL;
> +
> +	BUG_ON(!ss->use_id);
> +	rcu_read_lock();
> +	/* fill start point for scan */
> +	tmpid = id;
> +	while (1) {
> +		/*
> +		 * scan next entry from bitmap(tree), tmpid is updated after
> +		 * idr_get_next().
> +		 */
> +		spin_lock(&ss->id_lock);
> +		tmp = idr_get_next(&ss->idr, &tmpid);
> +		spin_unlock(&ss->id_lock);
> +
> +		if (!tmp)
> +			break;
> +		if (tmp->depth >= depth && tmp->stack[depth] == rootid) {
> +			ret = rcu_dereference(tmp->css);
> +			if (ret) {
> +				*foundid = tmpid;
> +				break;
> +			}
> +		}
> +		/* continue to scan from next id */
> +		tmpid = tmpid + 1;
> +	}
> +
> +	rcu_read_unlock();
> +	return ret;
> +}
> +
css_get_next is called under rcu_read_lock, so it doesn't need to call
rcu_read_lock/unlock by itself.

Otherwise, looks good to me.

	Reviewed-by: Daisuke Nishimura <nishimura@....nes.nec.co.jp>


Thanks,
Daisuke Nishimura.

On Thu, 22 Jan 2009 18:35:09 +0900, KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> 
> From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> 
> Patch for Per-CSS(Cgroup Subsys State) ID and private hierarchy code.
> 
> This patch attaches unique ID to each css and provides following.
> 
>  - css_lookup(subsys, id)
>    returns pointer to struct cgroup_subysys_state of id.
>  - css_get_next(subsys, id, rootid, depth, foundid)
>    returns the next css under "root" by scanning
> 
> When cgrou_subsys->use_id is set, an id for css is maintained.
> The cgroup framework only parepares
> 	- css_id of root css for subsys
> 	- id is automatically attached at creation of css.
> 	- id is *not* freed automatically. Because the cgroup framework
> 	  don't know lifetime of cgroup_subsys_state.
> 	  free_css_id() function is provided. This must be called by subsys.
> 
> There are several reasons to develop this.
> 	- Saving space .... For example, memcg's swap_cgroup is array of
> 	  pointers to cgroup. But it is not necessary to be very fast.
> 	  By replacing pointers(8bytes per ent) to ID (2byes per ent), we can
> 	  reduce much amount of memory usage.
> 
> 	- Scanning without lock.
> 	  CSS_ID provides "scan id under this ROOT" function. By this, scanning
> 	  css under root can be written without locks.
> 	  ex)
> 	  do {
> 		rcu_read_lock();
> 		next = cgroup_get_next(subsys, id, root, &found);
> 		/* check sanity of next here */
> 		css_tryget();
> 		rcu_read_unlock();
> 		id = found + 1
> 	 } while(...)
> 
> Characteristics: 
> 	- Each css has unique ID under subsys.
> 	- Lifetime of ID is controlled by subsys.
> 	- css ID contains "ID" and "Depth in hierarchy" and stack of hierarchy
> 	- Allowed ID is 1-65535, ID 0 is UNUSED ID.
> 
> Design Choices:
> 	- scan-by-ID v.s. scan-by-tree-walk.
> 	  As /proc's pid scan does, scan-by-ID is robust when scanning is done
> 	  by following kind of routine.
> 	  scan -> rest a while(release a lock) -> conitunue from interrupted
> 	  memcg's hierarchical reclaim does this.
> 
> 	- When subsys->use_id is set, # of css in the system is limited to
> 	  65535. 
> 
> Changelog: (v7) -> (v8)
> 	- Update id->css pointer after cgroup is populated.
> 
> Changelog: (v6) -> (v7)
> 	- refcnt for CSS ID is removed. Subsys can do it by own logic.
> 	- New id allocation is done automatically.
> 	- fixed typos.
> 	- fixed limit check of ID.
> 
> Changelog: (v5) -> (v6)
>  	- max depth is removed.
> 	- changed arguments to "scan"
> Changelog: (v4) -> (v5)
> 	- Totally re-designed as per-css ID.
> Changelog:(v3) -> (v4)
> 	- updated comments.
> 	- renamed hierarchy_code[] to stack[]
> 	- merged prepare_id routines.
> 
> Changelog (v2) -> (v3)
> 	- removed cgroup_id_getref().
> 	- added cgroup_id_tryget().
> 
> Changelog (v1) -> (v2):
> 	- Design change: show only ID(integer) to outside of cgroup.c
> 	- moved cgroup ID definition from include/ to kernel/cgroup.c
> 	- struct cgroup_id is freed by RCU.
> 	- changed interface from pointer to "int"
> 	- kill_sb() is handled. 
> 	- ID 0 as unused ID.
> 
> Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> ---
>  include/linux/cgroup.h |   50 ++++++++
>  include/linux/idr.h    |    1 
>  kernel/cgroup.c        |  289 ++++++++++++++++++++++++++++++++++++++++++++++++-
>  lib/idr.c              |   46 +++++++
>  4 files changed, 385 insertions(+), 1 deletion(-)
> 
> Index: mmotm-2.6.29-Jan16/include/linux/cgroup.h
> ===================================================================
> --- mmotm-2.6.29-Jan16.orig/include/linux/cgroup.h
> +++ mmotm-2.6.29-Jan16/include/linux/cgroup.h
> @@ -15,6 +15,7 @@
>  #include <linux/cgroupstats.h>
>  #include <linux/prio_heap.h>
>  #include <linux/rwsem.h>
> +#include <linux/idr.h>
>  
>  #ifdef CONFIG_CGROUPS
>  
> @@ -22,6 +23,7 @@ struct cgroupfs_root;
>  struct cgroup_subsys;
>  struct inode;
>  struct cgroup;
> +struct css_id;
>  
>  extern int cgroup_init_early(void);
>  extern int cgroup_init(void);
> @@ -59,6 +61,8 @@ struct cgroup_subsys_state {
>  	atomic_t refcnt;
>  
>  	unsigned long flags;
> +	/* ID for this css, if possible */
> +	struct css_id *id;
>  };
>  
>  /* bits in struct cgroup_subsys_state flags field */
> @@ -363,6 +367,11 @@ struct cgroup_subsys {
>  	int active;
>  	int disabled;
>  	int early_init;
> +	/*
> +	 * True if this subsys uses ID. ID is not available before cgroup_init()
> +	 * (not available in early_init time.)
> +	 */
> +	bool use_id;
>  #define MAX_CGROUP_TYPE_NAMELEN 32
>  	const char *name;
>  
> @@ -384,6 +393,9 @@ struct cgroup_subsys {
>  	 */
>  	struct cgroupfs_root *root;
>  	struct list_head sibling;
> +	/* used when use_id == true */
> +	struct idr idr;
> +	spinlock_t id_lock;
>  };
>  
>  #define SUBSYS(_x) extern struct cgroup_subsys _x ## _subsys;
> @@ -437,6 +449,44 @@ void cgroup_iter_end(struct cgroup *cgrp
>  int cgroup_scan_tasks(struct cgroup_scanner *scan);
>  int cgroup_attach_task(struct cgroup *, struct task_struct *);
>  
> +/*
> + * CSS ID is ID for cgroup_subsys_state structs under subsys. This only works
> + * if cgroup_subsys.use_id == true. It can be used for looking up and scanning.
> + * CSS ID is assigned at cgroup allocation (create) automatically
> + * and removed when subsys calls free_css_id() function. This is because
> + * the lifetime of cgroup_subsys_state is subsys's matter.
> + *
> + * Looking up and scanning function should be called under rcu_read_lock().
> + * Taking cgroup_mutex()/hierarchy_mutex() is not necessary for following calls.
> + * But the css returned by this routine can be "not populated yet" or "being
> + * destroyed". The caller should check css and cgroup's status.
> + */
> +
> +/*
> + * Typically Called at ->destroy(), or somewhere the subsys frees
> + * cgroup_subsys_state.
> + */
> +void free_css_id(struct cgroup_subsys *ss, struct cgroup_subsys_state *css);
> +
> +/* Find a cgroup_subsys_state which has given ID */
> +
> +struct cgroup_subsys_state *css_lookup(struct cgroup_subsys *ss, int id);
> +
> +/*
> + * Get a cgroup whose id is greater than or equal to id under tree of root.
> + * Returning a cgroup_subsys_state or NULL.
> + */
> +struct cgroup_subsys_state *css_get_next(struct cgroup_subsys *ss, int id,
> +		struct cgroup_subsys_state *root, int *foundid);
> +
> +/* Returns true if root is ancestor of cg */
> +bool css_is_ancestor(struct cgroup_subsys_state *cg,
> +		     struct cgroup_subsys_state *root);
> +
> +/* Get id and depth of css */
> +unsigned short css_id(struct cgroup_subsys_state *css);
> +unsigned short css_depth(struct cgroup_subsys_state *css);
> +
>  #else /* !CONFIG_CGROUPS */
>  
>  static inline int cgroup_init_early(void) { return 0; }
> Index: mmotm-2.6.29-Jan16/kernel/cgroup.c
> ===================================================================
> --- mmotm-2.6.29-Jan16.orig/kernel/cgroup.c
> +++ mmotm-2.6.29-Jan16/kernel/cgroup.c
> @@ -94,7 +94,6 @@ struct cgroupfs_root {
>  	char release_agent_path[PATH_MAX];
>  };
>  
> -
>  /*
>   * The "rootnode" hierarchy is the "dummy hierarchy", reserved for the
>   * subsystems that are otherwise unattached - it never has more than a
> @@ -102,6 +101,39 @@ struct cgroupfs_root {
>   */
>  static struct cgroupfs_root rootnode;
>  
> +/*
> + * CSS ID -- ID per subsys's Cgroup Subsys State(CSS). used only when
> + * cgroup_subsys->use_id != 0.
> + */
> +#define CSS_ID_MAX	(65535)
> +struct css_id {
> +	/*
> +	 * The css to which this ID points. This pointer is set to valid value
> +	 * after cgroup is populated. If cgroup is removed, this will be NULL.
> +	 * This pointer is expected to be RCU-safe because destroy()
> +	 * is called after synchronize_rcu(). But for safe use, css_is_removed()
> +	 * css_tryget() should be used for avoiding race.
> +	 */
> +	struct cgroup_subsys_state *css;
> +	/*
> +	 * ID of this css.
> +	 */
> +	unsigned short id;
> +	/*
> +	 * Depth in hierarchy which this ID belongs to.
> +	 */
> +	unsigned short depth;
> +	/*
> +	 * ID is freed by RCU. (and lookup routine is RCU safe.)
> +	 */
> +	struct rcu_head rcu_head;
> +	/*
> +	 * Hierarchy of CSS ID belongs to.
> +	 */
> +	unsigned short stack[0]; /* Array of Length (depth+1) */
> +};
> +
> +
>  /* The list of hierarchy roots */
>  
>  static LIST_HEAD(roots);
> @@ -185,6 +217,8 @@ struct cg_cgroup_link {
>  static struct css_set init_css_set;
>  static struct cg_cgroup_link init_css_set_link;
>  
> +static int cgroup_subsys_init_idr(struct cgroup_subsys *ss);
> +
>  /* css_set_lock protects the list of css_set objects, and the
>   * chain of tasks off each css_set.  Nests outside task->alloc_lock
>   * due to cgroup_iter_start() */
> @@ -567,6 +601,9 @@ static struct backing_dev_info cgroup_ba
>  	.capabilities	= BDI_CAP_NO_ACCT_AND_WRITEBACK,
>  };
>  
> +static int alloc_css_id(struct cgroup_subsys *ss,
> +			struct cgroup *parent, struct cgroup *child);
> +
>  static struct inode *cgroup_new_inode(mode_t mode, struct super_block *sb)
>  {
>  	struct inode *inode = new_inode(sb);
> @@ -2324,6 +2361,17 @@ static int cgroup_populate_dir(struct cg
>  		if (ss->populate && (err = ss->populate(ss, cgrp)) < 0)
>  			return err;
>  	}
> +	/* This cgroup is ready now */
> +	for_each_subsys(cgrp->root, ss) {
> +		struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
> +		/*
> +		 * Update id->css pointer and make this css visible from
> +		 * CSS ID functions. This pointer will be dereferened
> +		 * from RCU-read-side without locks.
> +		 */
> +		if (css->id)
> +			rcu_assign_pointer(css->id->css, css);
> +	}
>  
>  	return 0;
>  }
> @@ -2335,6 +2383,7 @@ static void init_cgroup_css(struct cgrou
>  	css->cgroup = cgrp;
>  	atomic_set(&css->refcnt, 1);
>  	css->flags = 0;
> +	css->id = NULL;
>  	if (cgrp == dummytop)
>  		set_bit(CSS_ROOT, &css->flags);
>  	BUG_ON(cgrp->subsys[ss->subsys_id]);
> @@ -2410,6 +2459,10 @@ static long cgroup_create(struct cgroup 
>  			goto err_destroy;
>  		}
>  		init_cgroup_css(css, ss, cgrp);
> +		if (ss->use_id)
> +			if (alloc_css_id(ss, parent, cgrp))
> +				goto err_destroy;
> +		/* At error, ->destroy() callback has to free assigned ID. */
>  	}
>  
>  	cgroup_lock_hierarchy(root);
> @@ -2701,6 +2754,8 @@ int __init cgroup_init(void)
>  		struct cgroup_subsys *ss = subsys[i];
>  		if (!ss->early_init)
>  			cgroup_init_subsys(ss);
> +		if (ss->use_id)
> +			cgroup_subsys_init_idr(ss);
>  	}
>  
>  	/* Add init_css_set to the hash table */
> @@ -3234,3 +3289,235 @@ static int __init cgroup_disable(char *s
>  	return 1;
>  }
>  __setup("cgroup_disable=", cgroup_disable);
> +
> +/*
> + * Functons for CSS ID.
> + */
> +
> +/*
> + *To get ID other than 0, this should be called when !cgroup_is_removed().
> + */
> +unsigned short css_id(struct cgroup_subsys_state *css)
> +{
> +	struct css_id *cssid = rcu_dereference(css->id);
> +
> +	if (cssid)
> +		return cssid->id;
> +	return 0;
> +}
> +
> +unsigned short css_depth(struct cgroup_subsys_state *css)
> +{
> +	struct css_id *cssid = rcu_dereference(css->id);
> +
> +	if (cssid)
> +		return cssid->depth;
> +	return 0;
> +}
> +
> +bool css_is_ancestor(struct cgroup_subsys_state *child,
> +		    struct cgroup_subsys_state *root)
> +{
> +	struct css_id *child_id = rcu_dereference(child->id);
> +	struct css_id *root_id = rcu_dereference(root->id);
> +
> +	if (!child_id || !root_id || (child_id->depth < root_id->depth))
> +		return false;
> +	return child_id->stack[root_id->depth] == root_id->id;
> +}
> +
> +static void __free_css_id_cb(struct rcu_head *head)
> +{
> +	struct css_id *id;
> +
> +	id = container_of(head, struct css_id, rcu_head);
> +	kfree(id);
> +}
> +
> +void free_css_id(struct cgroup_subsys *ss, struct cgroup_subsys_state *css)
> +{
> +	struct css_id *id = css->id;
> +	/* When this is called before css_id initialization, id can be NULL */
> +	if (!id)
> +		return;
> +
> +	BUG_ON(!ss->use_id);
> +
> +	rcu_assign_pointer(id->css, NULL);
> +	rcu_assign_pointer(css->id, NULL);
> +	spin_lock(&ss->id_lock);
> +	idr_remove(&ss->idr, id->id);
> +	spin_unlock(&ss->id_lock);
> +	call_rcu(&id->rcu_head, __free_css_id_cb);
> +}
> +
> +/*
> + * This is called by init or create(). Then, calls to this function are
> + * always serialized (By cgroup_mutex() at create()).
> + */
> +
> +static struct css_id *get_new_cssid(struct cgroup_subsys *ss, int depth)
> +{
> +	struct css_id *newid;
> +	int myid, error, size;
> +
> +	BUG_ON(!ss->use_id);
> +
> +	size = sizeof(*newid) + sizeof(unsigned short) * (depth + 1);
> +	newid = kzalloc(size, GFP_KERNEL);
> +	if (!newid)
> +		return ERR_PTR(-ENOMEM);
> +	/* get id */
> +	if (unlikely(!idr_pre_get(&ss->idr, GFP_KERNEL))) {
> +		error = -ENOMEM;
> +		goto err_out;
> +	}
> +	spin_lock(&ss->id_lock);
> +	/* Don't use 0. allocates an ID of 1-65535 */
> +	error = idr_get_new_above(&ss->idr, newid, 1, &myid);
> +	spin_unlock(&ss->id_lock);
> +
> +	/* Returns error when there are no free spaces for new ID.*/
> +	if (error) {
> +		error = -ENOSPC;
> +		goto err_out;
> +	}
> +	if (myid > CSS_ID_MAX)
> +		goto remove_idr;
> +
> +	newid->id = myid;
> +	newid->depth = depth;
> +	return newid;
> +remove_idr:
> +	error = -ENOSPC;
> +	spin_lock(&ss->id_lock);
> +	idr_remove(&ss->idr, myid);
> +	spin_unlock(&ss->id_lock);
> +err_out:
> +	kfree(newid);
> +	return ERR_PTR(error);
> +
> +}
> +
> +static int __init cgroup_subsys_init_idr(struct cgroup_subsys *ss)
> +{
> +	struct css_id *newid;
> +	struct cgroup_subsys_state *rootcss;
> +
> +	spin_lock_init(&ss->id_lock);
> +	idr_init(&ss->idr);
> +
> +	rootcss = init_css_set.subsys[ss->subsys_id];
> +	newid = get_new_cssid(ss, 0);
> +	if (IS_ERR(newid))
> +		return PTR_ERR(newid);
> +
> +	newid->stack[0] = newid->id;
> +	newid->css = rootcss;
> +	rootcss->id = newid;
> +	return 0;
> +}
> +
> +static int alloc_css_id(struct cgroup_subsys *ss, struct cgroup *parent,
> +			struct cgroup *child)
> +{
> +	int subsys_id, i, depth = 0;
> +	struct cgroup_subsys_state *parent_css, *child_css;
> +	struct css_id *child_id, *parent_id = NULL;
> +
> +	subsys_id = ss->subsys_id;
> +	parent_css = parent->subsys[subsys_id];
> +	child_css = child->subsys[subsys_id];
> +	depth = css_depth(parent_css) + 1;
> +	parent_id = parent_css->id;
> +
> +	child_id = get_new_cssid(ss, depth);
> +	if (IS_ERR(child_id))
> +		return PTR_ERR(child_id);
> +
> +	for (i = 0; i < depth; i++)
> +		child_id->stack[i] = parent_id->stack[i];
> +	child_id->stack[depth] = child_id->id;
> +	/*
> +	 * child_id->css pointer will be set after this cgroup is available
> +	 * see cgroup_populate_dir()
> +	 */
> +	rcu_assign_pointer(child_css->id, child_id);
> +
> +	return 0;
> +}
> +
> +/**
> + * css_lookup - lookup css by id
> + * @ss: cgroup subsys to be looked into.
> + * @id: the id
> + *
> + * Returns pointer to cgroup_subsys_state if there is valid one with id.
> + * NULL if not. Should be called under rcu_read_lock()
> + */
> +struct cgroup_subsys_state *css_lookup(struct cgroup_subsys *ss, int id)
> +{
> +	struct css_id *cssid = NULL;
> +
> +	BUG_ON(!ss->use_id);
> +	cssid = idr_find(&ss->idr, id);
> +
> +	if (unlikely(!cssid))
> +		return NULL;
> +
> +	return rcu_dereference(cssid->css);
> +}
> +
> +/**
> + * css_get_next - lookup next cgroup under specified hierarchy.
> + * @ss: pointer to subsystem
> + * @id: current position of iteration.
> + * @root: pointer to css. search tree under this.
> + * @foundid: position of found object.
> + *
> + * Search next css under the specified hierarchy of rootid. Calling under
> + * rcu_read_lock() is necessary. Returns NULL if it reaches the end.
> + */
> +struct cgroup_subsys_state *
> +css_get_next(struct cgroup_subsys *ss, int id,
> +	     struct cgroup_subsys_state *root, int *foundid)
> +{
> +	struct cgroup_subsys_state *ret = NULL;
> +	struct css_id *tmp;
> +	int tmpid;
> +	int rootid = css_id(root);
> +	int depth = css_depth(root);
> +
> +	if (!rootid)
> +		return NULL;
> +
> +	BUG_ON(!ss->use_id);
> +	rcu_read_lock();
> +	/* fill start point for scan */
> +	tmpid = id;
> +	while (1) {
> +		/*
> +		 * scan next entry from bitmap(tree), tmpid is updated after
> +		 * idr_get_next().
> +		 */
> +		spin_lock(&ss->id_lock);
> +		tmp = idr_get_next(&ss->idr, &tmpid);
> +		spin_unlock(&ss->id_lock);
> +
> +		if (!tmp)
> +			break;
> +		if (tmp->depth >= depth && tmp->stack[depth] == rootid) {
> +			ret = rcu_dereference(tmp->css);
> +			if (ret) {
> +				*foundid = tmpid;
> +				break;
> +			}
> +		}
> +		/* continue to scan from next id */
> +		tmpid = tmpid + 1;
> +	}
> +
> +	rcu_read_unlock();
> +	return ret;
> +}
> +
> Index: mmotm-2.6.29-Jan16/include/linux/idr.h
> ===================================================================
> --- mmotm-2.6.29-Jan16.orig/include/linux/idr.h
> +++ mmotm-2.6.29-Jan16/include/linux/idr.h
> @@ -106,6 +106,7 @@ int idr_get_new(struct idr *idp, void *p
>  int idr_get_new_above(struct idr *idp, void *ptr, int starting_id, int *id);
>  int idr_for_each(struct idr *idp,
>  		 int (*fn)(int id, void *p, void *data), void *data);
> +void *idr_get_next(struct idr *idp, int *nextid);
>  void *idr_replace(struct idr *idp, void *ptr, int id);
>  void idr_remove(struct idr *idp, int id);
>  void idr_remove_all(struct idr *idp);
> Index: mmotm-2.6.29-Jan16/lib/idr.c
> ===================================================================
> --- mmotm-2.6.29-Jan16.orig/lib/idr.c
> +++ mmotm-2.6.29-Jan16/lib/idr.c
> @@ -579,6 +579,52 @@ int idr_for_each(struct idr *idp,
>  EXPORT_SYMBOL(idr_for_each);
>  
>  /**
> + * idr_get_next - lookup next object of id to given id.
> + * @idp: idr handle
> + * @id:  pointer to lookup key
> + *
> + * Returns pointer to registered object with id, which is next number to
> + * given id.
> + */
> +
> +void *idr_get_next(struct idr *idp, int *nextidp)
> +{
> +	struct idr_layer *p, *pa[MAX_LEVEL];
> +	struct idr_layer **paa = &pa[0];
> +	int id = *nextidp;
> +	int n, max;
> +
> +	/* find first ent */
> +	n = idp->layers * IDR_BITS;
> +	max = 1 << n;
> +	p = rcu_dereference(idp->top);
> +	if (!p)
> +		return NULL;
> +
> +	while (id < max) {
> +		while (n > 0 && p) {
> +			n -= IDR_BITS;
> +			*paa++ = p;
> +			p = rcu_dereference(p->ary[(id >> n) & IDR_MASK]);
> +		}
> +
> +		if (p) {
> +			*nextidp = id;
> +			return p;
> +		}
> +
> +		id += 1 << n;
> +		while (n < fls(id)) {
> +			n += IDR_BITS;
> +			p = *--paa;
> +		}
> +	}
> +	return NULL;
> +}
> +
> +
> +
> +/**
>   * idr_replace - replace pointer for given id
>   * @idp: idr handle
>   * @ptr: pointer you want associated with the id
> 
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