linux-kernel - Re: [PATCH 3/3] mm: memcg: use non-unified stats flushing for userspace reads

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAJD7tkZsGfYXkWM5aa67v3JytTO04LS7_x+ooMDK82cBZ-C8eQ@mail.gmail.com>
Date:   Mon, 28 Aug 2023 10:28:44 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Waiman Long <longman@...hat.com>
Cc:     Shakeel Butt <shakeelb@...gle.com>, Michal Hocko <mhocko@...e.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        Muchun Song <muchun.song@...ux.dev>,
        Ivan Babrou <ivan@...udflare.com>, Tejun Heo <tj@...nel.org>,
        linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/3] mm: memcg: use non-unified stats flushing for
 userspace reads

On Mon, Aug 28, 2023 at 10:27 AM Waiman Long <longman@...hat.com> wrote:
>
>
> On 8/28/23 13:07, Yosry Ahmed wrote:
> >
> >> Here I agree with you. Let's go with the approach which is easy to
> >> undo for now. Though I prefer the new explicit interface for flushing,
> >> that step would be very hard to undo. Let's reevaluate if the proposed
> >> approach shows negative impact on production traffic and I think
> >> Cloudflare folks can give us the results soon.
> > Do you prefer we also switch to using a mutex (with preemption
> > disabled) to avoid the scenario Michal described where flushers give
> > up the lock and sleep resulting in an unbounded wait time in the worst
> > case?
>
> Locking with mutex with preemption disabled is an oxymoron. Use spinlock
> if you want to have preemption disabled. The purpose of usiing mutex is
> to allow the lock owner to sleep, but you can't sleep with preemption
> disabled. You need to enable preemption first. You can disable
> preemption for a short time in a non-sleeping section of the lock
> critical section, but I would not recommend disabling preemption for the
> whole critical section.

I thought using a mutex with preemption disabled would at least allow
waiters to sleep rather than spin, is this not correct (or doesn't
matter) ?

>
> Cheers,
> Longman
>