On Tue, 2020-10-27 at 21:30 +0100, Peter Zijlstra wrote: > On Tue, Oct 27, 2020 at 07:27:59PM +0000, David Woodhouse wrote: > > > > While looking at this I found that weird __add_wait_queue_exclusive() > > > which is used by fs/eventpoll.c and does something similar, except it > > > doesn't keep the FIFO order. > > > > It does, doesn't it? Except those so-called "exclusive" entries end up > > in FIFO order amongst themselves at the *tail* of the queue, to be > > woken up only after all the other entries before them *haven't* been > > excluded. > > __add_wait_queue_exclusive() uses __add_wait_queue() which does > list_add(). It does _not_ add at the tail like normal exclusive users, > and there is exactly _1_ user in tree that does this. > > I'm not exactly sure how this happened, but: > > add_wait_queue_exclusive() > > and > > __add_wait_queue_exclusive() > > are not related :-( I think that goes all the way back to here: https://lkml.org/lkml/2007/5/4/530 It was rounded up in commit d47de16c72and subsequently "cleaned up" into an inline in wait.h, but I don't think there was ever a reason for it to be added to the head of the list instead of the tail. So I think we can reasonably make __add_wait_queue_exclusive() do precisely the same thing as add_wait_queue_exclusive() does (modulo locking). And then potentially rename them both to something that isn't quite such a lie. And give me the one I want that *does* actually exclude other waiters :)
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