Re: [GIT PULL] iomap: new code for 5.4

[Linus Torvalds <torvalds@linux-foundation.org>]

On Tue, Sep 17, 2019 at 8:21 AM Darrick J. Wong <djwong@kernel.org> wrote:
>
> Please pull this series containing all the new iomap code for 5.4.

So looking at the non-iomap parts of it, I react to the new "list_pop() code.

In particular, this:

        struct list_head *pos = READ_ONCE(list->next);

is crazy to begin with..

It seems to have come from "list_empty()", but the difference is that
it actually makes sense to check for emptiness of a list outside
whatever lock that protects the list. It can be one of those very
useful optimizations where you don't even bother taking the lock if
you can optimistically check that the list is empty.

But the same is _not_ true of an operation like "list_pop()". By
definition, the list you pop something off has to be stable, so the
READ_ONCE() makes no sense here.

Anyway, if that was the only issue, I wouldn't care. But looking
closer, the whole thing is just completely wrong.

All the users seem to do some version of this:

        struct list_head tmp;

        list_replace_init(&ioend->io_list, &tmp);
        iomap_finish_ioend(ioend, error);
        while ((ioend = list_pop_entry(&tmp, struct iomap_ioend, io_list)))
                iomap_finish_ioend(ioend, error);

which is completely wrong and pointless.

Why would anybody use that odd "list_pop()" thing in a loop, when what
it really seems to just want is that bog-standard
"list_for_each_entry_safe()"

        struct list_head tmp;
        struct iomap_ioend *next;

        list_replace_init(&ioend->io_list, &tmp);
        iomap_finish_ioend(ioend, error);
        list_for_each_entry_safe(struct iomap_ioend, next, &tmp, io_list)
                iomap_finish_ioend(ioend, error);

which is not only the common pattern, it's more efficient and doesn't
pointlessly re-write the list for each entry, it just walks it (and
the "_safe()" part is because it looks up the next entry early, so
that the entry that it's walking can be deleted).

So I pulled it. But then after looking at it, I unpulled it again
because I don't want to see this kind of insanity in one of THE MOST
CORE header files we have in the whole kernel.

If xfs and iomap want to think they are "popping" a list, they can do
so. In the privacy of your own home, you can do stupid and pointless
things.

But no, we don't pollute core kernel code with those stupid and
pointless things.

              Linus