1 files changed, 71 insertions, 48 deletions

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index 21bba772235..2eade518660 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -63,6 +63,9 @@
 
 #include <linux/atomic.h>
 
+/* css deactivation bias, makes css->refcnt negative to deny new trygets */
+#define CSS_DEACT_BIAS		INT_MIN
+
 /*
  * cgroup_mutex is the master lock.  Any modification to cgroup or its
  * hierarchy must be performed while holding it.
@@ -251,6 +254,14 @@ int cgroup_lock_is_held(void)
 
 EXPORT_SYMBOL_GPL(cgroup_lock_is_held);
 
+/* the current nr of refs, always >= 0 whether @css is deactivated or not */
+static int css_refcnt(struct cgroup_subsys_state *css)
+{
+	int v = atomic_read(&css->refcnt);
+
+	return v >= 0 ? v : v - CSS_DEACT_BIAS;
+}
+
 /* convenient tests for these bits */
 inline int cgroup_is_removed(const struct cgroup *cgrp)
 {
@@ -4006,18 +4017,19 @@ static int cgroup_mkdir(struct inode *dir, struct dentry *dentry, umode_t mode)
 	return cgroup_create(c_parent, dentry, mode | S_IFDIR);
 }
 
+/*
+ * 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 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 we can
+ * be called via check_for_release() with no synchronization other than
+ * RCU, and the subsystem linked list isn't RCU-safe.
+ */
 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 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
-	 * we can be called via check_for_release() with no
-	 * synchronization other than RCU, and the subsystem linked
-	 * list isn't RCU-safe */
 	int i;
+
 	/*
 	 * We won't need to lock the subsys array, because the subsystems
 	 * we're concerned about aren't going anywhere since our cgroup root
@@ -4026,17 +4038,21 @@ static int cgroup_has_css_refs(struct cgroup *cgrp)
 	for (i = 0; i < CGROUP_SUBSYS_COUNT; i++) {
 		struct cgroup_subsys *ss = subsys[i];
 		struct cgroup_subsys_state *css;
+
 		/* Skip subsystems not present or not in this hierarchy */
 		if (ss == NULL || ss->root != cgrp->root)
 			continue;
+
 		css = cgrp->subsys[ss->subsys_id];
-		/* When called from check_for_release() it's possible
+		/*
+		 * When called from check_for_release() it's possible
 		 * that by this point the cgroup has been removed
 		 * and the css deleted. But a false-positive doesn't
 		 * 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) > 1))
+		 * release agent to be called anyway.
+		 */
+		if (css && css_refcnt(css) > 1)
 			return 1;
 	}
 	return 0;
@@ -4053,44 +4069,37 @@ static int cgroup_clear_css_refs(struct cgroup *cgrp)
 	struct cgroup_subsys *ss;
 	unsigned long flags;
 	bool failed = false;
+
 	local_irq_save(flags);
+
+	/*
+	 * Block new css_tryget() by deactivating refcnt.  If all refcnts
+	 * were 1 at the moment of deactivation, we succeeded.
+	 */
 	for_each_subsys(cgrp->root, ss) {
 		struct cgroup_subsys_state *css = cgrp->subsys[ss->subsys_id];
-		int refcnt;
-		while (1) {
-			/* 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
-			 */
-			if (atomic_cmpxchg(&css->refcnt, refcnt, 0) == refcnt)
-				break;
-			cpu_relax();
-		}
+
+		WARN_ON(atomic_read(&css->refcnt) < 0);
+		atomic_add(CSS_DEACT_BIAS, &css->refcnt);
+		failed |= css_refcnt(css) != 1;
 	}
- done:
+
+	/*
+	 * If succeeded, set REMOVED and put all the base refs; otherwise,
+	 * restore refcnts to positive values.  Either way, all in-progress
+	 * css_tryget() will be released.
+	 */
 	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 */
+
+		if (!failed) {
 			set_bit(CSS_REMOVED, &css->flags);
+			css_put(css);
+		} else {
+			atomic_sub(CSS_DEACT_BIAS, &css->refcnt);
 		}
 	}
+
 	local_irq_restore(flags);
 	return !failed;
 }
@@ -4887,13 +4896,28 @@ static void check_for_release(struct cgroup *cgrp)
 }
 
 /* Caller must verify that the css is not for root cgroup */
-void __css_put(struct cgroup_subsys_state *css, int count)
+bool __css_tryget(struct cgroup_subsys_state *css)
+{
+	do {
+		int v = css_refcnt(css);
+
+		if (atomic_cmpxchg(&css->refcnt, v, v + 1) == v)
+			return true;
+		cpu_relax();
+	} while (!test_bit(CSS_REMOVED, &css->flags));
+
+	return false;
+}
+EXPORT_SYMBOL_GPL(__css_tryget);
+
+/* Caller must verify that the css is not for root cgroup */
+void __css_put(struct cgroup_subsys_state *css)
 {
 	struct cgroup *cgrp = css->cgroup;
-	int val;
+
 	rcu_read_lock();
-	val = atomic_sub_return(count, &css->refcnt);
-	if (val == 1) {
+	atomic_dec(&css->refcnt);
+	if (css_refcnt(css) == 1) {
 		if (notify_on_release(cgrp)) {
 			set_bit(CGRP_RELEASABLE, &cgrp->flags);
 			check_for_release(cgrp);
@@ -4901,7 +4925,6 @@ void __css_put(struct cgroup_subsys_state *css, int count)
 		cgroup_wakeup_rmdir_waiter(cgrp);
 	}
 	rcu_read_unlock();
-	WARN_ON_ONCE(val < 1);
 }
 EXPORT_SYMBOL_GPL(__css_put);
 
@@ -5020,7 +5043,7 @@ unsigned short css_id(struct cgroup_subsys_state *css)
 	 * on this or this is under rcu_read_lock(). Once css->id is allocated,
 	 * it's unchanged until freed.
 	 */
-	cssid = rcu_dereference_check(css->id, atomic_read(&css->refcnt));
+	cssid = rcu_dereference_check(css->id, css_refcnt(css));
 
 	if (cssid)
 		return cssid->id;
@@ -5032,7 +5055,7 @@ unsigned short css_depth(struct cgroup_subsys_state *css)
 {
 	struct css_id *cssid;
 
-	cssid = rcu_dereference_check(css->id, atomic_read(&css->refcnt));
+	cssid = rcu_dereference_check(css->id, css_refcnt(css));
 
 	if (cssid)
 		return cssid->depth;