Rasmus Villemoes wrote: > On 05/03/2019 04.06, George Spelvin wrote: >> + * (Actually, it is always called with @a being the element which was >> + * originally first, so it is not necessary to to distinguish the @a < @b >> + * and @a == @b cases; the return value may be a simple boolean. But if >> + * you ever *use* this freedom, be sure to update this comment to document >> + * that code now depends on preserving this property!) > > This was and still is used at least by the block layer, and likely > others as well. While 3110fc79606fb introduced a bunch of if() return -1 > else if () ... stuff, it still ends with a 0/1 result. Before > 3110fc79606fb, it was even more obvious that this property was used.
Ah, thank you! I actually read through every list_sort caller in the kernel to see if I could find anywhere that used it and couldn't, but I didn't study this code carefully enough to see that it does in the last step. Since someone *does* use this, I'll change the comment signiicantly. > Grepping around shows that this could probably be used in more places, > gaining a cycle or two per cmp callback, e.g. xfs_buf_cmp. But that's of > course outside the scope of this series. The one that misled me at first was _xfs_buf_obj_cmp, which returns 0/1, but that's not used by list_sort(). xfs_buf_cmp returns -1/0/+1. As you might see from the comment around the cmp_func typedef, there are other things that could be cleaned up if we did a pass over all the call sites. (I'm almost tempted to tell the compiler than cmp_func is const, since it's supposed to be independent of the pointer frobbing that list_sort does, but then I remember Henry Spencer's maxim about lying to the compiler.)
