ucount: replace get_ucounts_or_wrap() with atomic_inc_not_zero()

get_ucounts_or_wrap() increments the counter and if the counter is
negative then it decrements it again in order to reset the previous
increment.  This statement can be replaced with atomic_inc_not_zero() to
only increment the counter if it is not yet 0.

This simplifies the get function because the put (if the get failed) can
be removed.  atomic_inc_not_zero() is implement as a cmpxchg() loop which
can be repeated several times if another get/put is performed in parallel.
This will be optimized later.

Increment the reference counter only if not yet dropped to zero.

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

kernel/ucount.c

@@ -146,25 +146,16 @@ static void hlist_add_ucounts(struct ucounts *ucounts)
 	spin_unlock_irq(&ucounts_lock);
 }
 
-static inline bool get_ucounts_or_wrap(struct ucounts *ucounts)
-{
-	/* Returns true on a successful get, false if the count wraps. */
-	return !atomic_add_negative(1, &ucounts->count);
-}
-
 struct ucounts *get_ucounts(struct ucounts *ucounts)
 {
-	if (!get_ucounts_or_wrap(ucounts)) {
-		put_ucounts(ucounts);
-		ucounts = NULL;
-	}
-	return 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_head *hashent = ucounts_hashentry(ns, uid);
-	bool wrapped;
 	struct ucounts *ucounts, *new = NULL;
 
 	spin_lock_irq(&ucounts_lock);
@@ -189,14 +180,11 @@ struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
 			return new;
 		}
 	}
-
-	wrapped = !get_ucounts_or_wrap(ucounts);
+	if (!atomic_inc_not_zero(&ucounts->count))
+		ucounts = NULL;
 	spin_unlock_irq(&ucounts_lock);
 	kfree(new);
-	if (wrapped) {
-		put_ucounts(ucounts);
-		return NULL;
-	}
+
 	return ucounts;
 }