Messages in this thread | | | Subject | Re: [PATCH v19 3/7] xbitmap: add more operations | From | Tetsuo Handa <> | Date | Wed, 13 Dec 2017 23:16:56 +0900 |
| |
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.
| |