Re: [PATCH v17 1/6] lib/xbitmap: Introduce xbitmap

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On 11/03/2017 06:55 PM, Tetsuo Handa wrote:
I'm commenting without understanding the logic.

Wei Wang wrote:
+
+bool xb_preload(gfp_t gfp);
+
Want __must_check annotation, for __radix_tree_preload() is marked
with __must_check annotation. By error failing to check result of
xb_preload() will lead to preemption kept disabled unexpectedly.


I don't disagree with this, but I find its wrappers, e.g. radix_tree_preload() and radix_tree_maybe_preload(), don't seem to have __must_chek added.



+int xb_set_bit(struct xb *xb, unsigned long bit)
+{
+	int err;
+	unsigned long index = bit / IDA_BITMAP_BITS;
+	struct radix_tree_root *root = &xb->xbrt;
+	struct radix_tree_node *node;
+	void **slot;
+	struct ida_bitmap *bitmap;
+	unsigned long ebit;
+
+	bit %= IDA_BITMAP_BITS;
+	ebit = bit + 2;
+
+	err = __radix_tree_create(root, index, 0, &node, &slot);
+	if (err)
+		return err;
+	bitmap = rcu_dereference_raw(*slot);
+	if (radix_tree_exception(bitmap)) {
+		unsigned long tmp = (unsigned long)bitmap;
+
+		if (ebit < BITS_PER_LONG) {
+			tmp |= 1UL << ebit;
+			rcu_assign_pointer(*slot, (void *)tmp);
+			return 0;
+		}
+		bitmap = this_cpu_xchg(ida_bitmap, NULL);
+		if (!bitmap)
Please write locking rules, in order to explain how memory
allocated by __radix_tree_create() will not leak.


For the memory allocated by __radix_tree_create(), I think we could add:

    if (!bitmap) {
        __radix_tree_delete(root, node, slot);
        break;
    }


For the locking rules, how about adding the following "Developer notes:" at the top of the file:

"
Locks are required to ensure that concurrent calls to xb_set_bit, xb_preload_and_set_bit, xb_test_bit, xb_clear_bit, xb_clear_bit_range, xb_find_next_set_bit and xb_find_next_zero_bit, for the same ida bitmap will not happen.
"

+bool xb_test_bit(struct xb *xb, unsigned long bit)
+{
+	unsigned long index = bit / IDA_BITMAP_BITS;
+	const struct radix_tree_root *root = &xb->xbrt;
+	struct ida_bitmap *bitmap = radix_tree_lookup(root, index);
+
+	bit %= IDA_BITMAP_BITS;
+
+	if (!bitmap)
+		return false;
+	if (radix_tree_exception(bitmap)) {
+		bit += RADIX_TREE_EXCEPTIONAL_SHIFT;
+		if (bit > BITS_PER_LONG)
Why not bit >= BITS_PER_LONG here?

Yes, I think it should be ">=" here. Thanks.

Best,
Wei
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