[PATCH-tip v5 08/21] futex: Allow direct attachment of futex_state objects to hash bucket

[Waiman Long]

Currently, the futex state objects can only be located indirectly as

        hash bucket => futex_q => futex state

Actually it can be beneficial in some cases to locate the futex state
object directly from the hash bucket without the futex_q middleman.
Therefore, a new list head to link the futex state objects as well
as a new spinlock to manage them are added to the hash bucket.

To limit size increase for UP systems, these new fields are only for
SMP machines where the cacheline alignment of the hash bucket leaves
it with enough empty space for the new fields.

Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
---
 kernel/futex.c | 24 ++++++++++++++++++++++++
 1 file changed, 24 insertions(+)

diff --git a/kernel/futex.c b/kernel/futex.c
index 928212c..2ced6c0 100644
--- a/kernel/futex.c
+++ b/kernel/futex.c
@@ -209,6 +209,11 @@ struct futex_state {
 	struct list_head list;
 
 	/*
+	 * Can link to fs_head in the owning hash bucket.
+	 */
+	struct list_head fs_list;
+
+	/*
 	 * The PI object:
 	 */
 	struct rt_mutex pi_mutex;
@@ -264,11 +269,24 @@ struct futex_q {
  * Hash buckets are shared by all the futex_keys that hash to the same
  * location.  Each key may have multiple futex_q structures, one for each task
  * waiting on a futex.
+ *
+ * Alternatively (in SMP), a key can be associated with a unique futex_state
+ * object where multiple waiters waiting for that futex can queue up in that
+ * futex_state object without using the futex_q structure. A separate
+ * futex_state lock (fs_lock) is used for processing those futex_state objects.
  */
 struct futex_hash_bucket {
 	atomic_t waiters;
 	spinlock_t lock;
 	struct plist_head chain;
+
+#ifdef CONFIG_SMP
+	/*
+	 * Fields for managing futex_state object list
+	 */
+	spinlock_t fs_lock;
+	struct list_head fs_head;
+#endif
 } ____cacheline_aligned_in_smp;
 
 /*
@@ -875,6 +893,8 @@ static int refill_futex_state_cache(void)
 		return -ENOMEM;
 
 	INIT_LIST_HEAD(&state->list);
+	INIT_LIST_HEAD(&state->fs_list);
+
 	/* pi_mutex gets initialized later */
 	state->owner = NULL;
 	atomic_set(&state->refcount, 1);
@@ -3365,6 +3385,10 @@ static int __init futex_init(void)
 		atomic_set(&futex_queues[i].waiters, 0);
 		plist_head_init(&futex_queues[i].chain);
 		spin_lock_init(&futex_queues[i].lock);
+#ifdef CONFIG_SMP
+		INIT_LIST_HEAD(&futex_queues[i].fs_head);
+		spin_lock_init(&futex_queues[i].fs_lock);
+#endif
 	}
 
 	return 0;
-- 
1.8.3.1