idr: add idr_layer->prefix

Add a field which carries the prefix of ID the idr_layer covers.  This
will be used to implement lookup hint.

This patch doesn't make use of the new field and doesn't introduce any
behavior difference.

Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/include/linux/idr.h b/include/linux/idr.h
index 43b87b1..7b1c5c6 100644 (file)
--- a/include/linux/idr.h
+++ b/include/linux/idr.h
@@ -28,6 +28,7 @@
 #define IDR_MASK ((1 << IDR_BITS)-1)
 
 struct idr_layer {
+       int                     prefix; /* the ID prefix of this idr_layer */
        DECLARE_BITMAP(bitmap, IDR_SIZE); /* A zero bit means "space here" */
        struct idr_layer __rcu  *ary[1<<IDR_BITS];
        int                     count;  /* When zero, we can release it */

diff --git a/lib/idr.c b/lib/idr.c
index d66e75b..5cd6029 100644 (file)
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -60,6 +60,16 @@ static int idr_max(int layers)
        return (1 << bits) - 1;
 }
 
+/*
+ * Prefix mask for an idr_layer at @layer.  For layer 0, the prefix mask is
+ * all bits except for the lower IDR_BITS.  For layer 1, 2 * IDR_BITS, and
+ * so on.
+ */
+static int idr_layer_prefix_mask(int layer)
+{
+       return ~idr_max(layer + 1);
+}
+
 static struct idr_layer *get_from_free_list(struct idr *idp)
 {
        struct idr_layer *p;
@@ -272,6 +282,7 @@ static int sub_alloc(struct idr *idp, int *starting_id, struct idr_layer **pa,
                        if (!new)
                                return -ENOMEM;
                        new->layer = l-1;
+                       new->prefix = id & idr_layer_prefix_mask(new->layer);
                        rcu_assign_pointer(p->ary[m], new);
                        p->count++;
                }
@@ -313,6 +324,7 @@ build_up:
                         * upwards.
                         */
                        p->layer++;
+                       WARN_ON_ONCE(p->prefix);
                        continue;
                }
                if (!(new = idr_layer_alloc(gfp_mask, layer_idr))) {
@@ -334,6 +346,7 @@ build_up:
                new->ary[0] = p;
                new->count = 1;
                new->layer = layers-1;
+               new->prefix = id & idr_layer_prefix_mask(new->layer);
                if (bitmap_full(p->bitmap, IDR_SIZE))
                        __set_bit(0, new->bitmap);
                p = new;