linux-kernel - Re: [CFT][PATCH] ucounts: Fix signal ucount refcounting

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Date:   Mon, 18 Oct 2021 00:25:54 -0600
From:   Yu Zhao <yuzhao@...gle.com>
To:     Rune Kleveland <rune.kleveland@...omedia.dk>,
        "Eric W. Biederman" <ebiederm@...ssion.com>
Cc:     Alexey Gladkov <legion@...nel.org>,
        Jordan Glover <Golden_Miller83@...tonmail.ch>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>,
        "containers\\@lists.linux-foundation.org" 
        <containers@...ts.linux-foundation.org>
Subject: Re: [CFT][PATCH] ucounts: Fix signal ucount refcounting

On Sun, Oct 17, 2021 at 10:47 AM Rune Kleveland
<rune.kleveland@...omedia.dk> wrote:
>
> Hi!
>
> After applying the below patch, the 5 most problematic servers have run
> without any issues for 23 hours. That never happened before the patch on
> 5.14, so the patch seems to have fixed the issue for me.

Confirm. I couldn't reproduce the problem on 5.14 either.

> On Monday there will be more load on the servers, which caused them to
> crash faster without the patch. I will let you know if it happens again.
>
> Best regards,
> Rune
>
> On 16/10/2021 00:10, Eric W. Biederman wrote:
> >
> > In commit fda31c50292a ("signal: avoid double atomic counter
> > increments for user accounting") Linus made a clever optimization to
> > how rlimits and the struct user_struct.  Unfortunately that
> > optimization does not work in the obvious way when moved to nested
> > rlimits.  The problem is that the last decrement of the per user
> > namespace per user sigpending counter might also be the last decrement
> > of the sigpending counter in the parent user namespace as well.  Which
> > means that simply freeing the leaf ucount in __free_sigqueue is not
> > enough.
> >
> > Maintain the optimization and handle the tricky cases by introducing
> > inc_rlimit_get_ucounts and dec_rlimit_put_ucounts.
> >
> > By moving the entire optimization into functions that perform all of
> > the work it becomes possible to ensure that every level is handled
> > properly.
> >
> > I wish we had a single user across all of the threads whose rlimit
> > could be charged so we did not need this complexity.