ucount: use rcuref_t for reference counting

Use rcuref_t for reference counting.  This eliminates the cmpxchg loop in
the get and put path.  This also eliminates the need to acquire the lock
in the put path because once the final user returns the reference, it can
no longer be obtained anymore.

Use rcuref_t for reference counting.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Sebastian Andrzej Siewior <[email protected]>
Reviewed-by: Paul E. McKenney <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Cc: Boqun Feng <[email protected]>
Cc: Joel Fernandes <[email protected]>
Cc: Josh Triplett <[email protected]>
Cc: Lai jiangshan <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Mengen Sun <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: "Uladzislau Rezki (Sony)" <[email protected]>
Cc: YueHong Wu <[email protected]>
Cc: Zqiang <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: Sebastian Andrzej Siewior

include/linux/user_namespace.h

@@ -8,6 +8,7 @@
 #include <linux/rculist_nulls.h>
 #include <linux/sched.h>
 #include <linux/workqueue.h>
+#include <linux/rcuref.h>
 #include <linux/rwsem.h>
 #include <linux/sysctl.h>
 #include <linux/err.h>
@@ -120,7 +121,7 @@ struct ucounts {
 	struct user_namespace *ns;
 	kuid_t uid;
 	struct rcu_head rcu;
-	atomic_t count;
+	rcuref_t count;
 	atomic_long_t ucount[UCOUNT_COUNTS];
 	atomic_long_t rlimit[UCOUNT_RLIMIT_COUNTS];
 };
@@ -133,9 +134,15 @@ void retire_userns_sysctls(struct user_namespace *ns);
 struct ucounts *inc_ucount(struct user_namespace *ns, kuid_t uid, enum ucount_type type);
 void dec_ucount(struct ucounts *ucounts, enum ucount_type type);
 struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid);
-struct ucounts * __must_check get_ucounts(struct ucounts *ucounts);
 void put_ucounts(struct ucounts *ucounts);
 
+static inline struct ucounts * __must_check get_ucounts(struct ucounts *ucounts)
+{
+	if (rcuref_get(&ucounts->count))
+		return ucounts;
+	return NULL;
+}
+
 static inline long get_rlimit_value(struct ucounts *ucounts, enum rlimit_type type)
 {
 	return atomic_long_read(&ucounts->rlimit[type]);

kernel/ucount.c

@@ -11,7 +11,7 @@
 struct ucounts init_ucounts = {
 	.ns    = &init_user_ns,
 	.uid   = GLOBAL_ROOT_UID,
-	.count = ATOMIC_INIT(1),
+	.count = RCUREF_INIT(1),
 };
 
 #define UCOUNTS_HASHTABLE_BITS 10
@@ -138,7 +138,7 @@ static struct ucounts *find_ucounts(struct user_namespace *ns, kuid_t uid,
 	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))
+			if (rcuref_get(&ucounts->count))
 				return ucounts;
 		}
 	}
@@ -154,13 +154,6 @@ static void hlist_add_ucounts(struct ucounts *ucounts)
 	spin_unlock_irq(&ucounts_lock);
 }
 
-struct ucounts *get_ucounts(struct ucounts *ucounts)
-{
-	if (atomic_inc_not_zero(&ucounts->count))
-		return ucounts;
-	return NULL;
-}
-
 struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
 {
 	struct hlist_nulls_head *hashent = ucounts_hashentry(ns, uid);
@@ -176,7 +169,7 @@ struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
 
 	new->ns = ns;
 	new->uid = uid;
-	atomic_set(&new->count, 1);
+	rcuref_init(&new->count, 1);
 
 	spin_lock_irq(&ucounts_lock);
 	ucounts = find_ucounts(ns, uid, hashent);
@@ -196,7 +189,8 @@ void put_ucounts(struct ucounts *ucounts)
 {
 	unsigned long flags;
 
-	if (atomic_dec_and_lock_irqsave(&ucounts->count, &ucounts_lock, flags)) {
+	if (rcuref_put(&ucounts->count)) {
+		spin_lock_irqsave(&ucounts_lock, flags);
 		hlist_nulls_del_rcu(&ucounts->node);
 		spin_unlock_irqrestore(&ucounts_lock, flags);