Re: [PATCH 8/8] xarray: Don't clear marks in xas_store()

[John Hubbard <jhubbard@nvidia.com>]

On 2/5/20 8:28 PM, 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.


aha, the key point. Thanks for explaining it.

thanks,
-- 
John Hubbard
NVIDIA

> 
> 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.
>