Re: [CFT][PATCH] ucounts: Fix signal ucount refcounting

Yu Zhao
2021-10-18T06:26:07Z

On Sun, Oct 17, 2021 at 10:47 AM Rune Kleveland
<rune.kleveland@infomedia.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.