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SubjectRe: [PATCH v19 3/7] xbitmap: add more operations
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Wei Wang wrote:
> On 12/12/2017 09:20 PM, Tetsuo Handa wrote:
> > Wei Wang wrote:
> >> +void xb_clear_bit_range(struct xb *xb, unsigned long start, unsigned long end)
> >> +{
> >> + struct radix_tree_root *root = &xb->xbrt;
> >> + struct radix_tree_node *node;
> >> + void **slot;
> >> + struct ida_bitmap *bitmap;
> >> + unsigned int nbits;
> >> +
> >> + for (; start < end; start = (start | (IDA_BITMAP_BITS - 1)) + 1) {
> >> + unsigned long index = start / IDA_BITMAP_BITS;
> >> + unsigned long bit = start % IDA_BITMAP_BITS;
> >> +
> >> + bitmap = __radix_tree_lookup(root, index, &node, &slot);
> >> + if (radix_tree_exception(bitmap)) {
> >> + unsigned long ebit = bit + 2;
> >> + unsigned long tmp = (unsigned long)bitmap;
> >> +
> >> + nbits = min(end - start + 1, BITS_PER_LONG - ebit);
> >> +
> >> + if (ebit >= BITS_PER_LONG)
> > What happens if we hit this "continue;" when "index == ULONG_MAX / IDA_BITMAP_BITS" ?
>
> Thanks. I also improved the test case for this. I plan to change the
> implementation a little bit to avoid such overflow (has passed the test
> case that I have, just post out for another set of eyes):
>
> {
> ...
> unsigned long idx = start / IDA_BITMAP_BITS;
> unsigned long bit = start % IDA_BITMAP_BITS;
> unsigned long idx_end = end / IDA_BITMAP_BITS;
> unsigned long ret;
>
> for (idx = start / IDA_BITMAP_BITS; idx <= idx_end; idx++) {
> unsigned long ida_start = idx * IDA_BITMAP_BITS;
>
> bitmap = __radix_tree_lookup(root, idx, &node, &slot);
> if (radix_tree_exception(bitmap)) {
> unsigned long tmp = (unsigned long)bitmap;
> unsigned long ebit = bit + 2;
>
> if (ebit >= BITS_PER_LONG)
> continue;

Will you please please do eliminate exception path?
I can't interpret what "ebit >= BITS_PER_LONG" means.
The reason you "continue;" is that all bits beyond are "0", isn't it?
Then, it would make sense to "continue;" when finding next "1" because
all bits beyond are "0". But how does it make sense to "continue;" when
finding next "0" despite all bits beyond are "0"?

> if (set)
> ret = find_next_bit(&tmp,
> BITS_PER_LONG, ebit);
> else
> ret = find_next_zero_bit(&tmp,
> BITS_PER_LONG,
> ebit);
> if (ret < BITS_PER_LONG)
> return ret - 2 + ida_start;
> } else if (bitmap) {
> if (set)
> ret = find_next_bit(bitmap->bitmap,
> IDA_BITMAP_BITS, bit);
> else
> ret = find_next_zero_bit(bitmap->bitmap,
> IDA_BITMAP_BITS, bit);

"bit" may not be 0 for the first round and "bit" is always 0 afterwords.
But where is the guaranteed that "end" is a multiple of IDA_BITMAP_BITS ?
Please explain why it is correct to use IDA_BITMAP_BITS unconditionally
for the last round.

> if (ret < IDA_BITMAP_BITS)
> return ret + ida_start;
> } else if (!bitmap && !set) {

At this point bitmap == NULL is guaranteed. Thus, "!bitmap && " is pointless.

> return bit + IDA_BITMAP_BITS * idx;
> }
> bit = 0;
> }
>
> return end;
> }
>
>



> >
> >> +/**
> >> + * xb_find_next_set_bit - find the next set bit in a range
> >> + * @xb: the xbitmap to search
> >> + * @start: the start of the range, inclusive
> >> + * @end: the end of the range, exclusive
> >> + *
> >> + * Returns: the index of the found bit, or @end + 1 if no such bit is found.
> >> + */
> >> +unsigned long xb_find_next_set_bit(struct xb *xb, unsigned long start,
> >> + unsigned long end)
> >> +{
> >> + return xb_find_next_bit(xb, start, end, 1);
> >> +}
> > Won't "exclusive" loose ability to handle ULONG_MAX ? Since this is a
> > library module, missing ability to handle ULONG_MAX sounds like an omission.
> > Shouldn't we pass (or return) whether "found or not" flag (e.g. strtoul() in
> > C library function)?
> >
> > bool xb_find_next_set_bit(struct xb *xb, unsigned long start, unsigned long end, unsigned long *result);
> > unsigned long xb_find_next_set_bit(struct xb *xb, unsigned long start, unsigned long end, bool *found);
>
> Yes, ULONG_MAX needs to be tested by xb_test_bit(). Compared to checking
> the return value, would it be the same to let the caller check for the
> ULONG_MAX boundary?
>

Why the caller needs to care about whether it is ULONG_MAX or not?

Also, one more thing you need to check. Have you checked how long does
xb_find_next_set_bit(xb, 0, ULONG_MAX) on an empty xbitmap takes?
If it causes soft lockup warning, should we add cond_resched() ?
If yes, you have to document that this API might sleep. If no, you
have to document that the caller of this API is responsible for
not to pass such a large value range.

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