[PATCH 1/2] hugepage: Protect region tracking lists with its own spinlock

[Anton Blanchard <anton@samba.org>]

In preparation for creating a hash of spinlocks to replace the global
hugetlb_instantiation_mutex, protect the region tracking code with
its own spinlock.

Signed-off-by: Anton Blanchard <anton@samba.org> 
---

The old code locked it with either:

	down_write(&mm->mmap_sem);
or
	down_read(&mm->mmap_sem);
	mutex_lock(&hugetlb_instantiation_mutex);

I chose to keep things simple and wrap everything with a single lock.
Do we need the parallelism the old code had in the down_write case?


Index: powerpc.git/mm/hugetlb.c
===================================================================
--- powerpc.git.orig/mm/hugetlb.c	2011-01-07 12:50:52.090440484 +1100
+++ powerpc.git/mm/hugetlb.c	2011-01-07 12:52:03.922704453 +1100
@@ -56,16 +56,6 @@ static DEFINE_SPINLOCK(hugetlb_lock);
 /*
  * Region tracking -- allows tracking of reservations and instantiated pages
  *                    across the pages in a mapping.
- *
- * The region data structures are protected by a combination of the mmap_sem
- * and the hugetlb_instantion_mutex.  To access or modify a region the caller
- * must either hold the mmap_sem for write, or the mmap_sem for read and
- * the hugetlb_instantiation mutex:
- *
- * 	down_write(&mm->mmap_sem);
- * or
- * 	down_read(&mm->mmap_sem);
- * 	mutex_lock(&hugetlb_instantiation_mutex);
  */
 struct file_region {
 	struct list_head link;
@@ -73,10 +63,14 @@ struct file_region {
 	long to;
 };
 
+static DEFINE_SPINLOCK(region_lock);
+
 static long region_add(struct list_head *head, long f, long t)
 {
 	struct file_region *rg, *nrg, *trg;
 
+	spin_lock(&region_lock);
+
 	/* Locate the region we are either in or before. */
 	list_for_each_entry(rg, head, link)
 		if (f <= rg->to)
@@ -106,6 +100,7 @@ static long region_add(struct list_head
 	}
 	nrg->from = f;
 	nrg->to = t;
+	spin_unlock(&region_lock);
 	return 0;
 }
 
@@ -114,6 +109,8 @@ static long region_chg(struct list_head
 	struct file_region *rg, *nrg;
 	long chg = 0;
 
+	spin_lock(&region_lock);
+
 	/* Locate the region we are before or in. */
 	list_for_each_entry(rg, head, link)
 		if (f <= rg->to)
@@ -124,14 +121,17 @@ static long region_chg(struct list_head
 	 * size such that we can guarantee to record the reservation. */
 	if (&rg->link == head || t < rg->from) {
 		nrg = kmalloc(sizeof(*nrg), GFP_KERNEL);
-		if (!nrg)
-			return -ENOMEM;
+		if (!nrg) {
+			chg = -ENOMEM;
+			goto out;
+		}
 		nrg->from = f;
 		nrg->to   = f;
 		INIT_LIST_HEAD(&nrg->link);
 		list_add(&nrg->link, rg->link.prev);
 
-		return t - f;
+		chg = t - f;
+		goto out;
 	}
 
 	/* Round our left edge to the current segment if it encloses us. */
@@ -144,7 +144,7 @@ static long region_chg(struct list_head
 		if (&rg->link == head)
 			break;
 		if (rg->from > t)
-			return chg;
+			goto out;
 
 		/* We overlap with this area, if it extends futher than
 		 * us then we must extend ourselves.  Account for its
@@ -155,6 +155,9 @@ static long region_chg(struct list_head
 		}
 		chg -= rg->to - rg->from;
 	}
+out:
+
+	spin_unlock(&region_lock);
 	return chg;
 }
 
@@ -163,12 +166,16 @@ static long region_truncate(struct list_
 	struct file_region *rg, *trg;
 	long chg = 0;
 
+	spin_lock(&region_lock);
+
 	/* Locate the region we are either in or before. */
 	list_for_each_entry(rg, head, link)
 		if (end <= rg->to)
 			break;
-	if (&rg->link == head)
-		return 0;
+	if (&rg->link == head) {
+		chg = 0;
+		goto out;
+	}
 
 	/* If we are in the middle of a region then adjust it. */
 	if (end > rg->from) {
@@ -185,6 +192,9 @@ static long region_truncate(struct list_
 		list_del(&rg->link);
 		kfree(rg);
 	}
+
+out:
+	spin_unlock(&region_lock);
 	return chg;
 }
 
@@ -193,6 +203,8 @@ static long region_count(struct list_hea
 	struct file_region *rg;
 	long chg = 0;
 
+	spin_lock(&region_lock);
+
 	/* Locate each segment we overlap with, and count that overlap. */
 	list_for_each_entry(rg, head, link) {
 		int seg_from;
@@ -209,6 +221,7 @@ static long region_count(struct list_hea
 		chg += seg_to - seg_from;
 	}
 
+	spin_unlock(&region_lock);
 	return chg;
 }