[PATCH v9 3/8] Use atomic_t for ucounts reference counting

[Alexey Gladkov]

The current implementation of the ucounts reference counter requires the
use of spin_lock. We're going to use get_ucounts() in more performance
critical areas like a handling of RLIMIT_SIGPENDING.

Now we need to use spin_lock only if we want to change the hashtable.

v9:
* Use a negative value to check that the ucounts->count is close to
  overflow.

Signed-off-by: Alexey Gladkov <gladkov.ale...@gmail.com>
---
 include/linux/user_namespace.h |  4 +--
 kernel/ucount.c                | 53 ++++++++++++----------------------
 2 files changed, 21 insertions(+), 36 deletions(-)

diff --git a/include/linux/user_namespace.h b/include/linux/user_namespace.h
index f71b5a4a3e74..d84cc2c0b443 100644
--- a/include/linux/user_namespace.h
+++ b/include/linux/user_namespace.h
@@ -92,7 +92,7 @@ struct ucounts {
        struct hlist_node node;
        struct user_namespace *ns;
        kuid_t uid;
-       int count;
+       atomic_t count;
        atomic_long_t ucount[UCOUNT_COUNTS];
 };
 
@@ -104,7 +104,7 @@ 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 *get_ucounts(struct ucounts *ucounts);
+struct ucounts * __must_check get_ucounts(struct ucounts *ucounts);
 void put_ucounts(struct ucounts *ucounts);
 
 #ifdef CONFIG_USER_NS
diff --git a/kernel/ucount.c b/kernel/ucount.c
index 50cc1dfb7d28..7bac19bb3f1e 100644
--- a/kernel/ucount.c
+++ b/kernel/ucount.c
@@ -11,7 +11,7 @@
 struct ucounts init_ucounts = {
        .ns    = &init_user_ns,
        .uid   = GLOBAL_ROOT_UID,
-       .count = 1,
+       .count = ATOMIC_INIT(1),
 };
 
 #define UCOUNTS_HASHTABLE_BITS 10
@@ -139,6 +139,15 @@ static void hlist_add_ucounts(struct ucounts *ucounts)
        spin_unlock_irq(&ucounts_lock);
 }
 
+struct ucounts *get_ucounts(struct ucounts *ucounts)
+{
+       if (ucounts && atomic_add_negative(1, &ucounts->count)) {
+               atomic_dec(&ucounts->count);
+               ucounts = NULL;
+       }
+       return ucounts;
+}
+
 struct ucounts *alloc_ucounts(struct user_namespace *ns, kuid_t uid)
 {
        struct hlist_head *hashent = ucounts_hashentry(ns, uid);
@@ -155,7 +164,7 @@ struct ucounts *alloc_ucounts(struct user_namespace *ns, 
kuid_t uid)
 
                new->ns = ns;
                new->uid = uid;
-               new->count = 0;
+               atomic_set(&new->count, 1);
 
                spin_lock_irq(&ucounts_lock);
                ucounts = find_ucounts(ns, uid, hashent);
@@ -163,33 +172,12 @@ struct ucounts *alloc_ucounts(struct user_namespace *ns, 
kuid_t uid)
                        kfree(new);
                } else {
                        hlist_add_head(&new->node, hashent);
-                       ucounts = new;
+                       spin_unlock_irq(&ucounts_lock);
+                       return new;
                }
        }
-       if (ucounts->count == INT_MAX)
-               ucounts = NULL;
-       else
-               ucounts->count += 1;
        spin_unlock_irq(&ucounts_lock);
-       return ucounts;
-}
-
-struct ucounts *get_ucounts(struct ucounts *ucounts)
-{
-       unsigned long flags;
-
-       if (!ucounts)
-               return NULL;
-
-       spin_lock_irqsave(&ucounts_lock, flags);
-       if (ucounts->count == INT_MAX) {
-               WARN_ONCE(1, "ucounts: counter has reached its maximum value");
-               ucounts = NULL;
-       } else {
-               ucounts->count += 1;
-       }
-       spin_unlock_irqrestore(&ucounts_lock, flags);
-
+       ucounts = get_ucounts(ucounts);
        return ucounts;
 }
 
@@ -197,15 +185,12 @@ void put_ucounts(struct ucounts *ucounts)
 {
        unsigned long flags;
 
-       spin_lock_irqsave(&ucounts_lock, flags);
-       ucounts->count -= 1;
-       if (!ucounts->count)
+       if (atomic_dec_and_test(&ucounts->count)) {
+               spin_lock_irqsave(&ucounts_lock, flags);
                hlist_del_init(&ucounts->node);
-       else
-               ucounts = NULL;
-       spin_unlock_irqrestore(&ucounts_lock, flags);
-
-       kfree(ucounts);
+               spin_unlock_irqrestore(&ucounts_lock, flags);
+               kfree(ucounts);
+       }
 }
 
 static inline bool atomic_long_inc_below(atomic_long_t *v, int u)
-- 
2.29.3