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Subjectsg_next() for struct scatterlist is confusing

according to the inline comments in include/linux/scatterlist.h:

"* If bit 0 is set, then the page_link contains a pointer to the next sg
* table list. Otherwise the next entry is at sg + 1."

but if that's the case, then the implementation of sg_next() seems a
bit weird:

* sg_next - return the next scatterlist entry in a list
* @sg: The current sg entry
* Description:
* Usually the next entry will be @sg@ + 1, but if this sg element is part
* of a chained scatterlist, it could jump to the start of a new
* scatterlist array.
static inline struct scatterlist *sg_next(struct scatterlist *sg)
BUG_ON(sg->sg_magic != SG_MAGIC);
if (sg_is_last(sg))
return NULL;

if (unlikely(sg_is_chain(sg)))
sg = sg_chain_ptr(sg);

return sg;

note how the comment says that the next entry will "usually" be
sg+1, "but" not if it's actually a pointer.

however, as i read the code above, sg is *always* incremented before
that testing. is that correct? am i just misreading something? or
could the comment have been a bit clearer?


Robert P. J. Day
Linux Consulting, Training and Annoying Kernel Pedantry
Waterloo, Ontario, CANADA
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