[Devel] [PATCH 1/7] introduce atomic_dec_and_lock_irqsave()

[Kirill Korotaev]

Oleg Nesterov noticed to me that the construction like
(used in beancounter patches and free_uid()):

  local_irq_save(flags);
  if (atomic_dec_and_lock(&refcnt, &lock))
	  ...

is not that good for preemtible kernels, since with preemption
spin_lock() can schedule() to reduce latency. However, it won't schedule
if interrupts are disabled.

So this patch introduces atomic_dec_and_lock_irqsave() as a logical
counterpart to atomic_dec_and_lock().

Signed-Off-By: Pavel Emelianov <xemul at sw.ru>
Signed-Off-By: Kirill Korotaev <dev at sw.ru>

---

 include/linux/spinlock.h |    6 ++++++
 kernel/user.c            |    5 +----
 lib/dec_and_lock.c       |   19 +++++++++++++++++++
 3 files changed, 26 insertions(+), 4 deletions(-)

--- ./include/linux/spinlock.h.dlirq	2006-08-28 10:17:35.000000000 +0400
+++ ./include/linux/spinlock.h	2006-08-28 11:22:37.000000000 +0400
@@ -266,6 +266,12 @@ extern int _atomic_dec_and_lock(atomic_t
 #define atomic_dec_and_lock(atomic, lock) \
 		__cond_lock(lock, _atomic_dec_and_lock(atomic, lock))
 
+extern int _atomic_dec_and_lock_irqsave(atomic_t *atomic, spinlock_t *lock,
+		unsigned long *flagsp);
+#define atomic_dec_and_lock_irqsave(atomic, lock, flags) \
+		__cond_lock(lock, \
+			_atomic_dec_and_lock_irqsave(atomic, lock, &flags))
+
 /**
  * spin_can_lock - would spin_trylock() succeed?
  * @lock: the spinlock in question.
--- ./kernel/user.c.dlirq	2006-07-10 12:39:20.000000000 +0400
+++ ./kernel/user.c	2006-08-28 11:08:56.000000000 +0400
@@ -108,15 +108,12 @@ void free_uid(struct user_struct *up)
 	if (!up)
 		return;
 
-	local_irq_save(flags);
-	if (atomic_dec_and_lock(&up->__count, &uidhash_lock)) {
+	if (atomic_dec_and_lock_irqsave(&up->__count, &uidhash_lock, flags)) {
 		uid_hash_remove(up);
 		spin_unlock_irqrestore(&uidhash_lock, flags);
 		key_put(up->uid_keyring);
 		key_put(up->session_keyring);
 		kmem_cache_free(uid_cachep, up);
-	} else {
-		local_irq_restore(flags);
 	}
 }
 
--- ./lib/dec_and_lock.c.dlirq	2006-04-21 11:59:36.000000000 +0400
+++ ./lib/dec_and_lock.c	2006-08-28 11:22:08.000000000 +0400
@@ -33,3 +33,22 @@ int _atomic_dec_and_lock(atomic_t *atomi
 }
 
 EXPORT_SYMBOL(_atomic_dec_and_lock);
+
+/*
+ * the same, but takes the lock with _irqsave
+ */
+int _atomic_dec_and_lock_irqsave(atomic_t *atomic, spinlock_t *lock,
+		unsigned long *flagsp)
+{
+#ifdef CONFIG_SMP
+	if (atomic_add_unless(atomic, -1, 1))
+		return 0;
+#endif
+	spin_lock_irqsave(lock, *flagsp);
+	if (atomic_dec_and_test(atomic))
+		return 1;
+	spin_unlock_irqrestore(lock, *flagsp);
+	return 0;
+}
+
+EXPORT_SYMBOL(_atomic_dec_and_lock_irqsave);