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

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Message-ID: <CAHk-=wgvGOnMF0ePU4xS236bOsP8jouj3rps+ysCaGXvCjh2Dg@mail.gmail.com>
Date:   Thu, 23 Jul 2020 16:43:08 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Hugh Dickins <hughd@...gle.com>
Cc:     Oleg Nesterov <oleg@...hat.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

On Thu, Jul 23, 2020 at 4:11 PM Hugh Dickins <hughd@...gle.com> wrote:
>
> On Thu, 23 Jul 2020, Linus Torvalds wrote:
> >
> > I'll send a new version after I actually test it.
>
> I'll give it a try when you're happy with it.

Ok, what I described is what I've been running for a while now. But I
don't put much stress on my system with my normal workload, so..

> I did try yesterday's
> with my swapping loads on home machines (3 of 4 survived 16 hours),
> and with some google stresstests on work machines (0 of 10 survived).
>
> I've not spent long analyzing the crashes, all of them in or below
> __wake_up_common() called from __wake_up_locked_key_bookmark():
> sometimes gets to run the curr->func() and crashes on something
> inside there (often list_del's lib/list_debug.c:53!), sometimes
> cannot get that far. Looks like the wait queue entries on the list
> were not entirely safe with that patch.

Hmm. The bug Oleg pointed out should be pretty theoretical. But I
think the new approach with WQ_FLAG_WOKEN was much better anyway,
despite me missing that one spot in the first version of the patch.

So here's two patches - the first one does that wake_page_function()
conversion, and the second one just does the memory ordering cleanup I
mentioned.

I don't think the second one shouldn't matter on x86, but who knows.

I don't enable list debugging, but I find list corruption surprising.
All of _that_ should be inside the page waiqueue lock, the only
unlocked part was the "list_empty_careful()" part.

But I'll walk over my patch mentally one more time. Here's the current
version, anyway.

                Linus

View attachment "0001-mm-rewrite-wait_on_page_bit_common-logic.patch" of type "text/x-patch" (6618 bytes)

View attachment "0002-list-add-list_del_init_careful-to-go-with-list_empty.patch" of type "text/x-patch" (2902 bytes)