linux-kernel - Re: [RFC PATCH] mm: silence soft lockups from unlock

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Date:   Sat, 25 Jul 2020 21:27:53 +0200
From:   Oleg Nesterov <oleg@...hat.com>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Hugh Dickins <hughd@...gle.com>, Michal Hocko <mhocko@...nel.org>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>
Subject: Re: [RFC PATCH] mm: silence soft lockups from unlock_page

Firstly, to avoid the confusion, let me repeat I think your patch is fine.

I too thought that non-exclusive waiters do not care about the bit state
and thus wake_page_function() can simply wake them all up.

But then I did "git blame", found your commit 3510ca20ece0150 and came to
conclusion there are reasons we should not do this.

On 07/25, Linus Torvalds wrote:
>
> On Sat, Jul 25, 2020 at 3:14 AM Oleg Nesterov <oleg@...hat.com> wrote:
> >
> > In essense, this partly reverts your commit 3510ca20ece0150
> > ("Minor page waitqueue cleanups"). I mean this part:
>
> Well, no. I mean, it keeps the "always add to the fail" behavior.

Ah, sorry for confusion, this doesn't matter. I didn't mean "fairness".

What I tried to say. AFAICS before that commit we had (almost) the same
behaviour you propose now: unlock_page/etc wakes all the non-exclusive
waiters up.

No?

Or I misunderstood your reply? Quite possibly, too late for me...

Oleg.