On Wed 22-07-20 11:29:20, Linus Torvalds wrote: > On Tue, Jul 21, 2020 at 8:33 AM Linus Torvalds > <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > > More likely, it's actually *caused* by that commit 11a19c7b099f, and > > what might be happening is that other CPU's are just adding new > > waiters to the list *while* we're waking things up, because somebody > > else already got the page lock again. > > > > Humor me.. Does something like this work instead? > > I went back and looked at this, because it bothered me. Thanks for pursuing this. I have learned that the affected system is in fact a production machine which doesn't seem to have any downtime window planned soon. Moreover the issue is not always reproducible. So I cannot guarantee I can have this or other patches tested soon which is really unfortunate. > And I'm no longer convinced it can possibly make a difference. > > Why? > > Because __wake_up_locked_key_bookmark() just calls __wake_up_common(), > and that one checks the return value of the wakeup function: > > ret = curr->func(curr, mode, wake_flags, key); > if (ret < 0) > break; > > and will not add the bookmark back to the list if this triggers. > > And the wakeup function does that same "stop walking" thing: > > if (test_bit(key->bit_nr, &key->page->flags)) > return -1; > > So if somebody else took the page lock, I think we should already have > stopped walking the list. Right! I didn't bother to look at the wakeup callback so have missed this. For completeness this behavior is there since 3510ca20ece01 which we have in our 4.12 based kernel as well. -- Michal Hocko SUSE Labs
