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From:   Yosry Ahmed <yosryahmed@google.com>
Date:   Thu, 30 Mar 2023 01:19:29 -0700
Message-ID: <CAJD7tkbjmBaXghQ+14Hy28r2LoWSim+LEjOPxaamYeA_kr2uVw@mail.gmail.com>
Subject: Re: [PATCH v2 4/9] cgroup: rstat: add WARN_ON_ONCE() if flushing
 outside task context
To:     Michal Hocko <mhocko@suse.com>
Cc:     Johannes Weiner <hannes@cmpxchg.org>,
        Shakeel Butt <shakeelb@google.com>, Tejun Heo <tj@kernel.org>,
        Josef Bacik <josef@toxicpanda.com>,
        Jens Axboe <axboe@kernel.dk>,
        Zefan Li <lizefan.x@bytedance.com>,
        Roman Gushchin <roman.gushchin@linux.dev>,
        Muchun Song <muchun.song@linux.dev>,
        Andrew Morton <akpm@linux-foundation.org>,
        =?UTF-8?Q?Michal_Koutn=C3=BD?= <mkoutny@suse.com>,
        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 30, 2023 at 1:15=E2=80=AFAM Michal Hocko <mhocko@suse.com> wrot=
e:
>
> On Thu 30-03-23 01:06:26, Yosry Ahmed wrote:
> [...]
> > If we achieve that, do you think it makes sense to add
> > WARN_ON_ONCE(irqs_disabled()) instead to prevent future users from
> > flushing while disabling irqs or in irq context?
>
> WARN_ON (similar to BUG_ON) will not prevent anybody from doing bad
> things. We already have means to shout about sleepable code being
> invoked from an atomic context and there is no reason to duplicate that.
> As I've said earlier WARN_ON might panic the system in some
> configurations (and yes they are used also in production systems - do
> not ask me why...). So please be careful about that and use that only
> when something really bad (yet recoverable) is going on.

Thanks for the information (I was about to ask why about production
systems, but okay..). I will avoid WARN_ON completely. For the
purposes of this series I will drop this patch anyway.

Any idea how to shout about "hey this may take too long, why are you
doing it with irqs disabled?!"?

>
> --
> Michal Hocko
> SUSE Labs