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

[Wei Wang]

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