On Wed 05-02-20 20:28:01, Matthew Wilcox wrote: > On Wed, Feb 05, 2020 at 07:48:57PM -0800, John Hubbard wrote: > > You can then set entries using xa_store() and get entries > > using xa_load(). xa_store will overwrite any entry with the > > new entry and return the previous entry stored at that index. You can > > use xa_erase(), instead of calling xa_store() with a > > ``NULL`` entry followed by xas_init_marks(). There is no difference between > > an entry that has never been stored to and one that has been erased. Those, > > in turn, are the same as an entry that has had ``NULL`` stored to it and > > also had its marks erased via xas_init_marks(). > > There's a fundamental misunderstanding here. If you store a NULL, the > marks go away. There is no such thing as a marked NULL entry. If you > observe such a thing, it can only exist through some kind of permitted > RCU race, and the entry must be ignored. If you're holding the xa_lock, > there is no way to observe a NULL entry with a search mark set. > > What Jan is trying to do is allow code that knows what it's doing > the ability to say "Skip clearing the marks for performance reasons. > The marks are already clear." > > I'm still mulling over the patches from Jan. There's something I don't > like about them, but I can't articulate it in a useful way yet. I'm on > board with the general principle, and obviously the xas_for_each_marked() > bug needs to be fixed. There are different ways how to look at what I'm doing :) I was thinking about it more like "xas_store() is for storing value at some index", "xas_erase() is when I want the value at some index removed from the data structure". Because these are principially different operations for any data structure (as much as erasing can be *implemented* by just storing NULL at some index). You seem to recognize this for xa_ functions but you probably considered xas_ functions internal enough that they follow more the "how it is implemented" way of thinking. Now I agree that there are holes in my way of thinking about xas_store() because if you happen to store NULL at some index, marks may get destroyed as a side-effect. And some users of __xa_cmpxchg() (BTW nobody seems to be using xa_cmpxchg_bh()) do use the fact that storing NULL does effectively erase an entry which is BTW inconsistent with xa_store() itself as well... You've been probably thinking more about xarray API semantics than I was so I can be convinced otherwise but at this point, I'd rather move the API more towards "erase is different from storing NULL". Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
