On Sat, Mar 19 2016, Jan Kara wrote:
>
> Actually, after some thought I don't think the wakeup is needed except for
> dax_pfn_mkwrite(). In the other cases we know there is no radix tree
> exceptional entry and thus there can be no waiters for its lock...
>

I think that is fragile logic - though it may be correct at present.

A radix tree slot can transition from "Locked exception" to "unlocked
exception" to "deleted" to "struct page".

So it is absolutely certain that a thread cannot go to sleep after
finding a "locked exception" and wake up to find a "struct page" ??

How about a much simpler change.
 - new local variable "slept" in lookup_unlocked_mapping_entry() which
   is set if prepare_to_wait_exclusive() gets called.
 - if after __radix_tree_lookup() returns:
        (ret==NULL || !radix_tree_exceptional_entry(ret)) && slept
   then it calls wakeup immediately - because if it was waiting,
   something else might be to.

That would cover all vaguely possible cases except dax_pfn_mkwrite()


Thanks,
NeilBrown