On Fri, 10 Feb 2012 23:25:42 +0400
Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx> wrote:
> This patch implements clean, simple and effective radix-tree iteration routine.
>
> Iterating divided into two phases:
> * lookup next chunk in radix-tree leaf node
> * iterating through slots in this chunk
>
> Main iterator function radix_tree_next_chunk() returns pointer to first slot,
> and stores in the struct radix_tree_iter index of next-to-last slot.
> For tagged-iterating it also constuct bitmask of tags for retunted chunk.
> All additional logic implemented as static-inline functions and macroses.
>
> Also patch adds radix_tree_find_next_bit() static-inline variant of
> find_next_bit() optimized for small constant size arrays, because
> find_next_bit() too heavy for searching in an array with one/two long elements.
>
> ...
>
> +
> +static inline
> +void **radix_tree_iter_init(struct radix_tree_iter *iter, unsigned long start)
Nit: if we're going to line break a function definition/declaration
line like this then the usual way is to split it before the function
name, so
static inline void **
radix_tree_iter_init(struct radix_tree_iter *iter, unsigned long start)
Old-school people did this so they could find the function with
/^radix_tree_iter_init in vi ;)
> +{
> + iter->index = 0; /* to bypass next_index overflow protection */
> + iter->next_index = start;
> + return NULL;
> +}
Why didn't it initialize .tags?
In fact .tags only ever gets initialized deep inside
radix_tree_next_chunk(), if !radix_tree_is_indirect_ptr(). Is this
correct?
>
> ...
>
> +/**
> + * radix_tree_next_slot - find next slot in chunk
> + *
> + * @slot pointer to slot
> + * @iter iterator state
> + * @flags RADIX_TREE_ITER_*
> + *
> + * Returns pointer to next slot, or NULL if no more left.
> + */
> +static __always_inline
> +void **radix_tree_next_slot(void **slot, struct radix_tree_iter *iter,
> + unsigned flags)
> +{
> + unsigned size, offset;
'offset' could be made local to the single code block which uses it.
personally I find that this leads to clearer code.
> + size = radix_tree_chunk_size(iter) - 1;
radix_tree_chunk_size() returns unsigned long, and we just threw away
the upper 32 bits. I'm unsure if that's a bug, but it's messy and
possibly inefficient.
> + if (flags & RADIX_TREE_ITER_TAGGED) {
> + iter->tags >>= 1;
> + if (likely(iter->tags & 1ul)) {
> + iter->index++;
> + return slot + 1;
> + }
> + if ((flags & RADIX_TREE_ITER_CONTIG) && size)
> + return NULL;
> + if (likely(iter->tags)) {
> + offset = __ffs(iter->tags);
> + iter->tags >>= offset;
> + iter->index += offset + 1;
> + return slot + offset + 1;
> + }
> + } else {
> + while (size--) {
> + slot++;
> + iter->index++;
> + if (likely(*slot))
> + return slot;
> + if (flags & RADIX_TREE_ITER_CONTIG)
> + return NULL;
> + }
> + }
> + return NULL;
> +}
This is a whopping big function. Why was it inlined? Are you sure
that was a correct decision?
> +/**
> + * radix_tree_for_each_chunk - iterate over chunks
> + *
> + * @slot: the void** for pointer to chunk first slot
> + * @root the struct radix_tree_root pointer
> + * @iter the struct radix_tree_iter pointer
> + * @start starting index
> + * @flags RADIX_TREE_ITER_* and tag index
Some of the arguments have a colon, others don't.
> + * Locks can be released and reasquired between iterations.
"reacquired"
> + */
> +#define radix_tree_for_each_chunk(slot, root, iter, start, flags) \
> + for ( slot = radix_tree_iter_init(iter, start) ; \
> + (slot = radix_tree_next_chunk(root, iter, flags)) ; )
I don't think I understand this whole interface :(
The term "chunk" has not been defined anywhere in the code, which
doesn't help.
Neither radix_tree_for_each_chunk() nor
radix_tree_for_each_chunk_slot() get used anywhere in this patchset so
one can't go look at call sites to work out what they're for.
It's a strange iterator - it never terminates. It requires that the
caller have an open-coded `break' in the search loop.
A bit more description and perhaps a usage example would help.
> +/**
> + * radix_tree_for_each_chunk_slot - iterate over slots in one chunk
> + *
> + * @slot: the void** for pointer to slot
> + * @iter the struct radix_tree_iter pointer
> + * @flags RADIX_TREE_ITER_*
> + */
> +#define radix_tree_for_each_chunk_slot(slot, iter, flags) \
> + for ( ; slot ; slot = radix_tree_next_slot(slot, iter, flags) )
Similar observations here.
> +/**
> + * radix_tree_for_each_slot - iterate over all slots
> + *
> + * @slot: the void** for pointer to slot
> + * @root the struct radix_tree_root pointer
> + * @iter the struct radix_tree_iter pointer
> + * @start starting index
> + */
> +#define radix_tree_for_each_slot(slot, root, iter, start) \
> + for ( slot = radix_tree_iter_init(iter, start) ; \
> + slot || (slot = radix_tree_next_chunk(root, iter, 0)) ; \
> + slot = radix_tree_next_slot(slot, iter, 0) )
All of these macros reference some of their arguments more than once.
So wierd and wrong things will happen if they are invoked with an
expression-with-side-effects. Also they lack parenthesisation, so
radix_tree_for_each_slot(myslot + 1, ...)
won't compile. The first problem is more serious than the second.
This is always a pain with complex macros and fixing it here would
deeply uglify the code. It's unlikely that anyone will be invoking
these with expression-with-side-effects so I'd be inclined to just live
with the dangers.
otoh, someone *might* do
radix_tree_for_each_slot(slot,
expensive_function_which_returns_a_root(),
iter, start);
and we'd call expensive_function_which_returns_a_root() each time
around the loop. But I don't think this is fixable.
Anyway, have a think about it all.
> +/**
> + * radix_tree_for_each_contig - iterate over all contiguous slots
Now what does this mean? Given a slot, iterate over that slot and all
contiguous successor slots until we encounter a hole?
Maybe. Again, better interface descriptions are needed, please.
> + * @slot: the void** for pointer to slot
> + * @root the struct radix_tree_root pointer
> + * @iter the struct radix_tree_iter pointer
> + * @start starting index
> + */
> +#define radix_tree_for_each_contig(slot, root, iter, start) \
> + for ( slot = radix_tree_iter_init(iter, start) ; \
> + slot || (slot = radix_tree_next_chunk(root, iter, \
> + RADIX_TREE_ITER_CONTIG)) ; \
> + slot = radix_tree_next_slot(slot, iter, \
> + RADIX_TREE_ITER_CONTIG) )
> +
> +/**
> + * radix_tree_for_each_tagged - iterate over all tagged slots
> + *
> + * @slot: the void** for pointer to slot
> + * @root the struct radix_tree_root pointer
> + * @iter the struct radix_tree_iter pointer
> + * @start starting index
> + * @tag tag index
> + */
> +#define radix_tree_for_each_tagged(slot, root, iter, start, tag) \
> + for ( slot = radix_tree_iter_init(iter, start) ; \
> + slot || (slot = radix_tree_next_chunk(root, iter, \
> + RADIX_TREE_ITER_TAGGED | tag)) ; \
> + slot = radix_tree_next_slot(slot, iter, \
> + RADIX_TREE_ITER_TAGGED) )
> +
> #endif /* _LINUX_RADIX_TREE_H */
>
> ...
>
> +static inline unsigned long radix_tree_find_next_bit(const unsigned long *addr,
> + unsigned long size, unsigned long offset)
> +{
> + if (!__builtin_constant_p(size))
> + return find_next_bit(addr, size, offset);
> +
> + if (offset < size) {
> + unsigned long tmp;
> +
> + addr += offset / BITS_PER_LONG;
> + tmp = *addr >> (offset % BITS_PER_LONG);
> + if (tmp)
> + return __ffs(tmp) + offset;
> + offset = (offset + BITS_PER_LONG) & ~(BITS_PER_LONG - 1);
> + while (offset < size) {
> + tmp = *++addr;
> + if (tmp)
> + return __ffs(tmp) + offset;
> + offset += BITS_PER_LONG;
> + }
> + }
> + return size;
> +}
Beware that gcc will freely ignore your "inline" directive.
When I compiled it, gcc did appear to inline it. Then I added
__always_inline and it was still inlined, but the text section in the
.o file got 20 bytes larger. Odd.
> /*
> * This assumes that the caller has performed appropriate preallocation, and
> * that the caller has pinned this thread of control to the current CPU.
> @@ -613,6 +649,117 @@ int radix_tree_tag_get(struct radix_tree_root *root,
> EXPORT_SYMBOL(radix_tree_tag_get);
>
> /**
> + * radix_tree_next_chunk - find next chunk of slots for iteration
> + *
> + * @root: radix tree root
> + * @iter: iterator state
> + * @flags RADIX_TREE_ITER_* flags and tag index
> + *
> + * Returns pointer to first slots in chunk, or NULL if there no more left
> + */
> +void **radix_tree_next_chunk(struct radix_tree_root *root,
> + struct radix_tree_iter *iter, unsigned flags)
> +{
> + unsigned shift, tag = flags & RADIX_TREE_ITER_TAG_MASK;
> + struct radix_tree_node *rnode, *node;
> + unsigned long i, index;
When a c programmer sees a variable called "i", he solidly expects it
to have type "int". Please choose a better name for this guy!
Perferably something which helps the reader understand what the
variable's role is.
> + if ((flags & RADIX_TREE_ITER_TAGGED) && !root_tag_get(root, tag))
> + return NULL;
> +
> + /*
> + * Catch next_index overflow after ~0UL.
> + * iter->index can be zero only at the beginning.
> + * Because RADIX_TREE_MAP_SHIFT < BITS_PER_LONG we cannot
> + * oveflow iter->next_index in single step.
> + */
> + index = iter->next_index;
> + if (!index && iter->index)
> + return NULL;
> +
> + rnode = rcu_dereference_raw(root->rnode);
> + if (radix_tree_is_indirect_ptr(rnode)) {
> + rnode = indirect_to_ptr(rnode);
> + } else if (rnode && !index) {
> + /* Single-slot tree */
> + iter->index = 0;
> + iter->next_index = 1;
> + iter->tags = 1;
> + return (void **)&root->rnode;
> + } else
> + return NULL;
> +
> +restart:
> + shift = (rnode->height - 1) * RADIX_TREE_MAP_SHIFT;
> + i = index >> shift;
> +
> + /* Index ouside of the tree */
> + if (i >= RADIX_TREE_MAP_SIZE)
> + return NULL;
> +
> + node = rnode;
> + while (1) {
> + if ((flags & RADIX_TREE_ITER_TAGGED) ?
> + !test_bit(i, node->tags[tag]) :
> + !node->slots[i]) {
> + /* Hole detected */
> + if (flags & RADIX_TREE_ITER_CONTIG)
> + return NULL;
> +
> + if (flags & RADIX_TREE_ITER_TAGGED)
> + i = radix_tree_find_next_bit(node->tags[tag],
> + RADIX_TREE_MAP_SIZE, i + 1);
> + else
> + while (++i < RADIX_TREE_MAP_SIZE &&
> + !node->slots[i]);
> +
> + index &= ~((RADIX_TREE_MAP_SIZE << shift) - 1);
> + index += i << shift;
> + /* Overflow after ~0UL */
> + if (!index)
> + return NULL;
> + if (i == RADIX_TREE_MAP_SIZE)
> + goto restart;
> + }
> +
> + /* This is leaf-node */
> + if (!shift)
> + break;
> +
> + node = rcu_dereference_raw(node->slots[i]);
> + if (node == NULL)
> + goto restart;
> + shift -= RADIX_TREE_MAP_SHIFT;
> + i = (index >> shift) & RADIX_TREE_MAP_MASK;
> + }
> +
> + /* Update the iterator state */
> + iter->index = index;
> + iter->next_index = (index | RADIX_TREE_MAP_MASK) + 1;
> +
> + /* Construct iter->tags bitmask from node->tags[tag] array */
> + if (flags & RADIX_TREE_ITER_TAGGED) {
> + unsigned tag_long, tag_bit;
> +
> + tag_long = i / BITS_PER_LONG;
> + tag_bit = i % BITS_PER_LONG;
> + iter->tags = node->tags[tag][tag_long] >> tag_bit;
> + /* This never happens if RADIX_TREE_TAG_LONGS == 1 */
> + if (tag_long < RADIX_TREE_TAG_LONGS - 1) {
> + /* Pick tags from next element */
> + if (tag_bit)
> + iter->tags |= node->tags[tag][tag_long + 1] <<
> + (BITS_PER_LONG - tag_bit);
> + /* Clip chunk size, here only BITS_PER_LONG tags */
> + iter->next_index = index + BITS_PER_LONG;
> + }
> + }
> +
> + return node->slots + i;
> +}
> +EXPORT_SYMBOL(radix_tree_next_chunk);
> +
> +/**
> * radix_tree_range_tag_if_tagged - for each item in given range set given
> * tag if item has another tag set
> * @root: radix tree root
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