Use a dedicated flag variable to track foffset locking.

Add part of the commit message into the comment.

Thanks for your review, Mark! I'll get to fixing the issues you highlighted soon.

The functional portion of the change seems fine, but I'm not sure why it's a problem for vm_page_grab* to support VM_ALLOC_WAITFAIL?

Move the comment.

foffset_lock1() has local scope.

Apply Gleb's suggestion.

In D49447#1127828, @zlei wrote:

Do I understand it right that the goal is for the output path to stop "seeing" an interface that is being detached earlier?

Yes. In D49359 I commented

@glebius

So instead of a lockless check for IFF_DYING

I think we eventually do not need IFF_DYING.

In D49447#1127724, @glebius wrote:

So instead of a lockless check for IFF_DYING you use lockless check for presence in STAILQ. I don't see a principal change here, but it could make the races less probable.

Thanks for the clarifications. Looks okay to me.

more feedback from bz

Simplify according to kib's suggestion.

Simplify control flow a bit.

Add foffset_trylock(), which can signal an error out-of-band in
addition to returning an offset.

Checking in to see if there is any movement on getting this reviewed/committed.

Try to improve comments.