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From:   Shakeel Butt <shakeelb@google.com>
Date:   Thu, 23 Mar 2023 08:46:34 -0700
Message-ID: <CALvZod7f9Rejb_WrZ+Ajegz-NsQ7iPQegRDMdk5Ya0a0w=40kg@mail.gmail.com>
Subject: Re: [RFC PATCH 1/7] cgroup: rstat: only disable interrupts for the
 percpu lock
To:     Yosry Ahmed <yosryahmed@google.com>
Cc:     Tejun Heo <tj@kernel.org>, Josef Bacik <josef@toxicpanda.com>,
        Jens Axboe <axboe@kernel.dk>,
        Zefan Li <lizefan.x@bytedance.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Michal Hocko <mhocko@kernel.org>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Muchun Song <muchun.song@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>,
        Vasily Averin <vasily.averin@linux.dev>,
        cgroups@vger.kernel.org, linux-block@vger.kernel.org,
        linux-kernel@vger.kernel.org, linux-mm@kvack.org,
        bpf@vger.kernel.org
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On Thu, Mar 23, 2023 at 8:43=E2=80=AFAM Yosry Ahmed <yosryahmed@google.com>=
 wrote:
>
> On Thu, Mar 23, 2023 at 8:40=E2=80=AFAM Shakeel Butt <shakeelb@google.com=
> wrote:
> >
> > On Thu, Mar 23, 2023 at 6:36=E2=80=AFAM Yosry Ahmed <yosryahmed@google.=
com> wrote:
> > >
> > [...]
> > > > >
> > > > > > 2. Are we really calling rstat flush in irq context?
> > > > >
> > > > > I think it is possible through the charge/uncharge path:
> > > > > memcg_check_events()->mem_cgroup_threshold()->mem_cgroup_usage().=
 I
> > > > > added the protection against flushing in an interrupt context for
> > > > > future callers as well, as it may cause a deadlock if we don't di=
sable
> > > > > interrupts when acquiring cgroup_rstat_lock.
> > > > >
> > > > > > 3. The mem_cgroup_flush_stats() call in mem_cgroup_usage() is o=
nly
> > > > > > done for root memcg. Why is mem_cgroup_threshold() interested i=
n root
> > > > > > memcg usage? Why not ignore root memcg in mem_cgroup_threshold(=
) ?
> > > > >
> > > > > I am not sure, but the code looks like event notifications may be=
 set
> > > > > up on root memcg, which is why we need to check thresholds.
> > > >
> > > > This is something we should deprecate as root memcg's usage is ill =
defined.
> > >
> > > Right, but I think this would be orthogonal to this patch series.
> > >
> >
> > I don't think we can make cgroup_rstat_lock a non-irq-disabling lock
> > without either breaking a link between mem_cgroup_threshold and
> > cgroup_rstat_lock or make mem_cgroup_threshold work without disabling
> > irqs.
> >
> > So, this patch can not be applied before either of those two tasks are
> > done (and we may find more such scenarios).
>
>
> Could you elaborate why?
>
> My understanding is that with an in_task() check to make sure we only
> acquire cgroup_rstat_lock from non-irq context it should be fine to
> acquire cgroup_rstat_lock without disabling interrupts.

From mem_cgroup_threshold() code path, cgroup_rstat_lock will be taken
with irq disabled while other code paths will take cgroup_rstat_lock
with irq enabled. This is a potential deadlock hazard unless
cgroup_rstat_lock is always taken with irq disabled.