xarray: Replace exceptional entries
Introduce xarray value entries and tagged pointers to replace radix
tree exceptional entries. This is a slight change in encoding to allow
the use of an extra bit (we can now store BITS_PER_LONG - 1 bits in a
value entry). It is also a change in emphasis; exceptional entries are
intimidating and different. As the comment explains, you can choose
to store values or pointers in the xarray and they are both first-class
citizens.
Signed-off-by: Matthew Wilcox <willy@infradead.org>
Reviewed-by: Josef Bacik <jbacik@fb.com>
diff --git a/lib/idr.c b/lib/idr.c
index 729e381..88419fb 100644
--- a/lib/idr.c
+++ b/lib/idr.c
@@ -338,11 +338,8 @@ EXPORT_SYMBOL(idr_replace);
* by the number of bits in the leaf bitmap before doing a radix tree lookup.
*
* As an optimisation, if there are only a few low bits set in any given
- * leaf, instead of allocating a 128-byte bitmap, we use the 'exceptional
- * entry' functionality of the radix tree to store BITS_PER_LONG - 2 bits
- * directly in the entry. By being really tricksy, we could store
- * BITS_PER_LONG - 1 bits, but there're diminishing returns after optimising
- * for 0-3 allocated IDs.
+ * leaf, instead of allocating a 128-byte bitmap, we store the bits
+ * directly in the entry.
*
* We allow the radix tree 'exceptional' count to get out of date. Nothing
* in the IDA nor the radix tree code checks it. If it becomes important
@@ -366,12 +363,11 @@ static int ida_get_new_above(struct ida *ida, int start)
struct radix_tree_iter iter;
struct ida_bitmap *bitmap;
unsigned long index;
- unsigned bit, ebit;
+ unsigned bit;
int new;
index = start / IDA_BITMAP_BITS;
bit = start % IDA_BITMAP_BITS;
- ebit = bit + RADIX_TREE_EXCEPTIONAL_SHIFT;
slot = radix_tree_iter_init(&iter, index);
for (;;) {
@@ -386,25 +382,23 @@ static int ida_get_new_above(struct ida *ida, int start)
return PTR_ERR(slot);
}
}
- if (iter.index > index) {
+ if (iter.index > index)
bit = 0;
- ebit = RADIX_TREE_EXCEPTIONAL_SHIFT;
- }
new = iter.index * IDA_BITMAP_BITS;
bitmap = rcu_dereference_raw(*slot);
- if (radix_tree_exception(bitmap)) {
- unsigned long tmp = (unsigned long)bitmap;
- ebit = find_next_zero_bit(&tmp, BITS_PER_LONG, ebit);
- if (ebit < BITS_PER_LONG) {
- tmp |= 1UL << ebit;
- rcu_assign_pointer(*slot, (void *)tmp);
- return new + ebit -
- RADIX_TREE_EXCEPTIONAL_SHIFT;
+ if (xa_is_value(bitmap)) {
+ unsigned long tmp = xa_to_value(bitmap);
+ int vbit = find_next_zero_bit(&tmp, BITS_PER_XA_VALUE,
+ bit);
+ if (vbit < BITS_PER_XA_VALUE) {
+ tmp |= 1UL << vbit;
+ rcu_assign_pointer(*slot, xa_mk_value(tmp));
+ return new + vbit;
}
bitmap = this_cpu_xchg(ida_bitmap, NULL);
if (!bitmap)
return -EAGAIN;
- bitmap->bitmap[0] = tmp >> RADIX_TREE_EXCEPTIONAL_SHIFT;
+ bitmap->bitmap[0] = tmp;
rcu_assign_pointer(*slot, bitmap);
}
@@ -425,17 +419,14 @@ static int ida_get_new_above(struct ida *ida, int start)
new += bit;
if (new < 0)
return -ENOSPC;
- if (ebit < BITS_PER_LONG) {
- bitmap = (void *)((1UL << ebit) |
- RADIX_TREE_EXCEPTIONAL_ENTRY);
- radix_tree_iter_replace(root, &iter, slot,
- bitmap);
- return new;
+ if (bit < BITS_PER_XA_VALUE) {
+ bitmap = xa_mk_value(1UL << bit);
+ } else {
+ bitmap = this_cpu_xchg(ida_bitmap, NULL);
+ if (!bitmap)
+ return -EAGAIN;
+ __set_bit(bit, bitmap->bitmap);
}
- bitmap = this_cpu_xchg(ida_bitmap, NULL);
- if (!bitmap)
- return -EAGAIN;
- __set_bit(bit, bitmap->bitmap);
radix_tree_iter_replace(root, &iter, slot, bitmap);
}
@@ -457,9 +448,9 @@ static void ida_remove(struct ida *ida, int id)
goto err;
bitmap = rcu_dereference_raw(*slot);
- if (radix_tree_exception(bitmap)) {
+ if (xa_is_value(bitmap)) {
btmp = (unsigned long *)slot;
- offset += RADIX_TREE_EXCEPTIONAL_SHIFT;
+ offset += 1; /* Intimate knowledge of the value encoding */
if (offset >= BITS_PER_LONG)
goto err;
} else {
@@ -470,9 +461,8 @@ static void ida_remove(struct ida *ida, int id)
__clear_bit(offset, btmp);
radix_tree_iter_tag_set(&ida->ida_rt, &iter, IDR_FREE);
- if (radix_tree_exception(bitmap)) {
- if (rcu_dereference_raw(*slot) ==
- (void *)RADIX_TREE_EXCEPTIONAL_ENTRY)
+ if (xa_is_value(bitmap)) {
+ if (xa_to_value(rcu_dereference_raw(*slot)) == 0)
radix_tree_iter_delete(&ida->ida_rt, &iter, slot);
} else if (bitmap_empty(btmp, IDA_BITMAP_BITS)) {
kfree(bitmap);
@@ -503,7 +493,7 @@ void ida_destroy(struct ida *ida)
xa_lock_irqsave(&ida->ida_rt, flags);
radix_tree_for_each_slot(slot, &ida->ida_rt, &iter, 0) {
struct ida_bitmap *bitmap = rcu_dereference_raw(*slot);
- if (!radix_tree_exception(bitmap))
+ if (!xa_is_value(bitmap))
kfree(bitmap);
radix_tree_iter_delete(&ida->ida_rt, &iter, slot);
}