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Date:   Thu, 23 Jul 2020 10:03:49 +0200
From:   Michal Hocko <mhocko@kernel.org>
To:     Linus Torvalds <torvalds@linux-foundation.org>
Cc:     Linux-MM <linux-mm@kvack.org>, LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        Tim Chen <tim.c.chen@linux.intel.com>
Subject: Re: [RFC PATCH] mm: silence soft lockups from unlock_page
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On Wed 22-07-20 11:29:20, Linus Torvalds wrote:
> On Tue, Jul 21, 2020 at 8:33 AM Linus Torvalds
> <torvalds@linux-foundation.org> 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