[PATCH 3/4] ucount: Use RCU for ucounts lookups.

[Sebastian Andrzej Siewior]

The ucounts element is looked up under ucounts_lock. This can be
optimized by using RCU for a lockless lookup and return and element if the
reference can be obtained.

Replace hlist_head with hlist_nulls_head which is RCU compatible. Let
find_ucounts() search for the required item within a RCU section and
return the item if a reference could be obtained.
This means alloc_ucounts() will always return an element (unless the
memory allocation failed). Let put_ucounts() RCU free the element if the
reference counter dropped to zero.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
---
 include/linux/user_namespace.h |  4 +-
 kernel/ucount.c                | 75 ++++++++++++++++++----------------
 2 files changed, 43 insertions(+), 36 deletions(-)

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index 7183e5aca2829..ad4dbef92597b 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -5,6 +5,7 @@
 #include <linux/kref.h>
 #include <linux/nsproxy.h>
 #include <linux/ns_common.h>
+#include <linux/rculist_nulls.h>
 #include <linux/sched.h>
 #include <linux/workqueue.h>
 #include <linux/rwsem.h>
@@ -115,9 +116,10 @@ struct user_namespace {
 } __randomize_layout;
 
 struct ucounts {
-	struct hlist_node node;
+	struct hlist_nulls_node node;
 	struct user_namespace *ns;
 	kuid_t uid;
+	struct rcu_head rcu;
 	atomic_t count;
 	atomic_long_t ucount[UCOUNT_COUNTS];
 	atomic_long_t rlimit[UCOUNT_RLIMIT_COUNTS];
diff --git a/kernel/ucount.c b/kernel/ucount.c
index 4aa5011538257..b6abaf68cdccb 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -15,7 +15,10 @@ struct ucounts init_ucounts = {
 };
 
 #define UCOUNTS_HASHTABLE_BITS 10
-static struct hlist_head ucounts_hashtable[(1 << UCOUNTS_HASHTABLE_BITS)];
+#define UCOUNTS_HASHTABLE_ENTRIES (1 << UCOUNTS_HASHTABLE_BITS)
+static struct hlist_nulls_head ucounts_hashtable[UCOUNTS_HASHTABLE_ENTRIES] = {
+	[0 ... UCOUNTS_HASHTABLE_ENTRIES - 1] = HLIST_NULLS_HEAD_INIT(0)
+};
 static DEFINE_SPINLOCK(ucounts_lock);
 
 #define ucounts_hashfn(ns, uid)						\
@@ -24,7 +27,6 @@ static DEFINE_SPINLOCK(ucounts_lock);
 #define ucounts_hashentry(ns, uid)	\
 	(ucounts_hashtable + ucounts_hashfn(ns, uid))
 
-
 #ifdef CONFIG_SYSCTL
 static struct ctl_table_set *
 set_lookup(struct ctl_table_root *root)
@@ -127,22 +129,28 @@ void retire_userns_sysctls(struct user_namespace *ns)
 #endif
 }
 
-static struct ucounts *find_ucounts(struct user_namespace *ns, kuid_t uid, struct hlist_head *hashent)
+static struct ucounts *find_ucounts(struct user_namespace *ns, kuid_t uid,
+				    struct hlist_nulls_head *hashent)
 {
 	struct ucounts *ucounts;
+	struct hlist_nulls_node *pos;
 
-	hlist_for_each_entry(ucounts, hashent, node) {
-		if (uid_eq(ucounts->uid, uid) && (ucounts->ns == ns))
-			return ucounts;
+	guard(rcu)();
+	hlist_nulls_for_each_entry_rcu(ucounts, pos, hashent, node) {
+		if (uid_eq(ucounts->uid, uid) && (ucounts->ns == ns)) {
+			if (atomic_inc_not_zero(&ucounts->count))
+				return ucounts;
+		}
 	}
 	return NULL;
 }
 
 static void hlist_add_ucounts(struct ucounts *ucounts)
 {
-	struct hlist_head *hashent = ucounts_hashentry(ucounts->ns, ucounts->uid);
+	struct hlist_nulls_head *hashent = ucounts_hashentry(ucounts->ns, ucounts->uid);
+
 	spin_lock_irq(&ucounts_lock);
-	hlist_add_head(&ucounts->node, hashent);
+	hlist_nulls_add_head_rcu(&ucounts->node, hashent);
 	spin_unlock_irq(&ucounts_lock);
 }
 
@@ -155,37 +163,33 @@ struct ucounts *get_ucounts(struct ucounts *ucounts)
 
 struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
 {
-	struct hlist_head *hashent = ucounts_hashentry(ns, uid);
-	struct ucounts *ucounts, *new = NULL;
+	struct hlist_nulls_head *hashent = ucounts_hashentry(ns, uid);
+	struct ucounts *ucounts, *new;
+
+	ucounts = find_ucounts(ns, uid, hashent);
+	if (ucounts)
+		return ucounts;
+
+	new = kzalloc(sizeof(*new), GFP_KERNEL);
+	if (!new)
+		return NULL;
+
+	new->ns = ns;
+	new->uid = uid;
+	atomic_set(&new->count, 1);
 
 	spin_lock_irq(&ucounts_lock);
 	ucounts = find_ucounts(ns, uid, hashent);
-	if (!ucounts) {
+	if (ucounts) {
 		spin_unlock_irq(&ucounts_lock);
-
-		new = kzalloc(sizeof(*new), GFP_KERNEL);
-		if (!new)
-			return NULL;
-
-		new->ns = ns;
-		new->uid = uid;
-		atomic_set(&new->count, 1);
-
-		spin_lock_irq(&ucounts_lock);
-		ucounts = find_ucounts(ns, uid, hashent);
-		if (!ucounts) {
-			hlist_add_head(&new->node, hashent);
-			get_user_ns(new->ns);
-			spin_unlock_irq(&ucounts_lock);
-			return new;
-		}
+		kfree(new);
+		return ucounts;
 	}
-	if (!atomic_inc_not_zero(&ucounts->count))
-		ucounts = NULL;
-	spin_unlock_irq(&ucounts_lock);
-	kfree(new);
 
-	return ucounts;
+	hlist_nulls_add_head_rcu(&new->node, hashent);
+	get_user_ns(new->ns);
+	spin_unlock_irq(&ucounts_lock);
+	return new;
 }
 
 void put_ucounts(struct ucounts *ucounts)
@@ -193,10 +197,11 @@ void put_ucounts(struct ucounts *ucounts)
 	unsigned long flags;
 
 	if (atomic_dec_and_lock_irqsave(&ucounts->count, &ucounts_lock, flags)) {
-		hlist_del_init(&ucounts->node);
+		hlist_nulls_del_rcu(&ucounts->node);
 		spin_unlock_irqrestore(&ucounts_lock, flags);
+
 		put_user_ns(ucounts->ns);
-		kfree(ucounts);
+		kfree_rcu(ucounts, rcu);
 	}
 }
 
-- 
2.47.2