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Commit e7c5ec91 authored by Paul Menage's avatar Paul Menage Committed by Linus Torvalds
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cgroups: add css_tryget() 



Add css_tryget(), that obtains a counted reference on a CSS.  It is used
in situations where the caller has a "weak" reference to the CSS, i.e.
one that does not protect the cgroup from removal via a reference count,
but would instead be cleaned up by a destroy() callback.

css_tryget() will return true on success, or false if the cgroup is being
removed.

This is similar to Kamezawa Hiroyuki's patch from a week or two ago, but
with the difference that in the event of css_tryget() racing with a
cgroup_rmdir(), css_tryget() will only return false if the cgroup really
does get removed.

This implementation is done by biasing css->refcnt, so that a refcnt of 1
means "releasable" and 0 means "released or releasing".  In the event of a
race, css_tryget() distinguishes between "released" and "releasing" by
checking for the CSS_REMOVED flag in css->flags.

Signed-off-by: default avatarPaul Menage <menage@google.com>
Tested-by: default avatarKAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Cc: Li Zefan <lizf@cn.fujitsu.com>
Cc: Balbir Singh <balbir@in.ibm.com>
Signed-off-by: default avatarAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 2cb378c8

include/linux/cgroup.h

+32 −6
Original line number Diff line number Diff line
@@ -52,9 +52,9 @@ struct cgroup_subsys_state { 
	 * hierarchy structure */

	struct cgroup *cgroup;



	/* State maintained by the cgroup system to allow

	 * subsystems to be "busy". Should be accessed via css_get()

	 * and css_put() */

	/* State maintained by the cgroup system to allow subsystems

	 * to be "busy". Should be accessed via css_get(),

	 * css_tryget() and and css_put(). */



	atomic_t refcnt;
@@ -64,11 +64,14 @@ struct cgroup_subsys_state { 
/* bits in struct cgroup_subsys_state flags field */

enum {

	CSS_ROOT, /* This CSS is the root of the subsystem */

	CSS_REMOVED, /* This CSS is dead */

};



/*

 * Call css_get() to hold a reference on the cgroup;

 *

 * Call css_get() to hold a reference on the css; it can be used

 * for a reference obtained via:

 * - an existing ref-counted reference to the css

 * - task->cgroups for a locked task

 */



static inline void css_get(struct cgroup_subsys_state *css)
@@ -77,9 +80,32 @@ static inline void css_get(struct cgroup_subsys_state *css) 
	if (!test_bit(CSS_ROOT, &css->flags))

		atomic_inc(&css->refcnt);

}



static inline bool css_is_removed(struct cgroup_subsys_state *css)

{

	return test_bit(CSS_REMOVED, &css->flags);

}



/*

 * Call css_tryget() to take a reference on a css if your existing

 * (known-valid) reference isn't already ref-counted. Returns false if

 * the css has been destroyed.

 */



static inline bool css_tryget(struct cgroup_subsys_state *css)

{

	if (test_bit(CSS_ROOT, &css->flags))

		return true;

	while (!atomic_inc_not_zero(&css->refcnt)) {

		if (test_bit(CSS_REMOVED, &css->flags))

			return false;

	}

	return true;

}



/*

 * css_put() should be called to release a reference taken by

 * css_get()

 * css_get() or css_tryget()

 */



extern void __css_put(struct cgroup_subsys_state *css);

kernel/cgroup.c

+56 −5
Original line number Diff line number Diff line
@@ -2333,7 +2333,7 @@ static void init_cgroup_css(struct cgroup_subsys_state *css, 
			       struct cgroup *cgrp)

{

	css->cgroup = cgrp;

	atomic_set(&css->refcnt, 0);

	atomic_set(&css->refcnt, 1);

	css->flags = 0;

	if (cgrp == dummytop)

		set_bit(CSS_ROOT, &css->flags);
@@ -2465,7 +2465,7 @@ static int cgroup_has_css_refs(struct cgroup *cgrp) 
{

	/* Check the reference count on each subsystem. Since we

	 * already established that there are no tasks in the

	 * cgroup, if the css refcount is also 0, then there should

	 * cgroup, if the css refcount is also 1, then there should

	 * be no outstanding references, so the subsystem is safe to

	 * destroy. We scan across all subsystems rather than using

	 * the per-hierarchy linked list of mounted subsystems since
@@ -2486,12 +2486,62 @@ static int cgroup_has_css_refs(struct cgroup *cgrp) 
		 * matter, since it can only happen if the cgroup

		 * has been deleted and hence no longer needs the

		 * release agent to be called anyway. */

		if (css && atomic_read(&css->refcnt))

		if (css && (atomic_read(&css->refcnt) > 1))

			return 1;

	}

	return 0;

}



/*

 * Atomically mark all (or else none) of the cgroup's CSS objects as

 * CSS_REMOVED. Return true on success, or false if the cgroup has

 * busy subsystems. Call with cgroup_mutex held

 */



static int cgroup_clear_css_refs(struct cgroup *cgrp)

{

	struct cgroup_subsys *ss;

	unsigned long flags;

	bool failed = false;

	local_irq_save(flags);

	for_each_subsys(cgrp->root, ss) {

		struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];

		int refcnt;

		do {

			/* We can only remove a CSS with a refcnt==1 */

			refcnt = atomic_read(&css->refcnt);

			if (refcnt > 1) {

				failed = true;

				goto done;

			}

			BUG_ON(!refcnt);

			/*

			 * Drop the refcnt to 0 while we check other

			 * subsystems. This will cause any racing

			 * css_tryget() to spin until we set the

			 * CSS_REMOVED bits or abort

			 */

		} while (atomic_cmpxchg(&css->refcnt, refcnt, 0) != refcnt);

	}

 done:

	for_each_subsys(cgrp->root, ss) {

		struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];

		if (failed) {

			/*

			 * Restore old refcnt if we previously managed

			 * to clear it from 1 to 0

			 */

			if (!atomic_read(&css->refcnt))

				atomic_set(&css->refcnt, 1);

		} else {

			/* Commit the fact that the CSS is removed */

			set_bit(CSS_REMOVED, &css->flags);

		}

	}

	local_irq_restore(flags);

	return !failed;

}



static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry)

{

	struct cgroup *cgrp = dentry->d_fsdata;
@@ -2522,7 +2572,7 @@ static int cgroup_rmdir(struct inode *unused_dir, struct dentry *dentry) 


	if (atomic_read(&cgrp->count)

	    || !list_empty(&cgrp->children)

	    || cgroup_has_css_refs(cgrp)) {

	    || !cgroup_clear_css_refs(cgrp)) {

		mutex_unlock(&cgroup_mutex);

		return -EBUSY;

	}
@@ -3078,7 +3128,8 @@ void __css_put(struct cgroup_subsys_state *css) 
{

	struct cgroup *cgrp = css->cgroup;

	rcu_read_lock();

	if (atomic_dec_and_test(&css->refcnt) && notify_on_release(cgrp)) {

	if ((atomic_dec_return(&css->refcnt) == 1) &&

	    notify_on_release(cgrp)) {

		set_bit(CGRP_RELEASABLE, &cgrp->flags);

		check_for_release(cgrp);

	}