Re: [PATCH 1/3] dmapool: improve scalability of dma_pool_alloc

[Tony Battersby <tonyb@cybernetics.com>]

> That would be true if the test were "if
> (list_empty(&pool->avail_page_list))".A  But it is testing the list
> pointers in the item rather than the list pointers in the pool.A  It may
> be a bit confusing if you have never seen that usage before, which is
> why I added a comment.A  Basically, if you use list_del_init() instead of
> list_del(), then you can use list_empty() on the item itself to test if
> the item is present in a list or not.A  For example, the comments in
> list.h warn not to use list_empty() on the entry after just list_del():
>
> /**
>  * list_del - deletes entry from list.
>  * @entry: the element to delete from the list.
>  * Note: list_empty() on entry does not return true after this, the entry is
>  * in an undefined state.
>  */

Sorry for the crappy line length (fixed above).A  Should have just used
Preformat in Thunderbird like always rather than following
Documentation/process/email-clients.rst and changing mailnews.wraplength
from 72 to 0.

Anyway, I have been using list_del_init() for a long time in various
places, but now I can't find where any of its useful features are
documented.A  I will have to submit a separate patch to add more
documentation for it.A  I find it useful for two things.

1) If you use list_del_init(), you can delete an item from a list
multiple times without checking if it has already been deleted.A  So the
following is safe:

list_add(entry, list);
list_del_init(entry);
list_del_init(entry);

That would not be safe if just using list_del().

2) If you use list_del_init(), you can test if an item is present in a
list using list_empty() on the entry.A  So:

list_add(entry, list);
/* list_empty(entry) is false */
list_del_init(entry);
/* list_empty(entry) is true */

That would not work if using just list_del().

Since the list_empty() name is unintuitive for this purpose, it might be
useful to add a new macro for this use case.