Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 2620:137:e000::1:20 as permitted sender) client-ip=2620:137:e000::1:20;
Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Wed, 3 Aug 2022 10:39:47 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Liu Song <liusong@linux.alibaba.com>
Cc:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, vschneid@redhat.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched/debug: avoid executing show_state and causing rcu
 stall warning
Message-ID: <Yuo0U2aWUZRLBAsA@gmail.com>
References: <1659489525-82994-1-git-send-email-liusong@linux.alibaba.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <1659489525-82994-1-git-send-email-liusong@linux.alibaba.com>
Precedence: bulk


* Liu Song <liusong@linux.alibaba.com> wrote:

> From: Liu Song <liusong@linux.alibaba.com>
> 
> If the number of CPUs is large, "sysrq_sched_debug_show" will execute for
> a long time. Every time I execute "echo t > /proc/sysrq-trigger" on my
> 128-core machine, the rcu stall warning will be triggered. Moreover,
> sysrq_sched_debug_show does not need to be protected by rcu_read_lock,
  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> and no rcu stall warning will appear after adjustment.
> 

That doesn't mean it doesn't have to be protected by *any* lock - which 
your patch implements AFAICS.

There's a couple of lines such as:

        for_each_online_cpu(cpu) {

... which need to be protected against CPU hotplug events.

I haven't checked any of the deeper code to see what RCU or other 
protection it may need, but clearly you didn't either ...

Thanks,

	Ingo