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Date:   Tue, 14 Sep 2021 21:06:27 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Pingfan Liu <kernelfans@gmail.com>
Cc:     linux-kernel@vger.kernel.org, Petr Mladek <pmladek@suse.com>,
        Wang Qing <wangqing@vivo.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Santosh Sivaraj <santosh@fossix.org>,
        Sumit Garg <sumit.garg@linaro.org>,
        Will Deacon <will@kernel.org>,
        Mark Rutland <mark.rutland@arm.com>
Subject: Re: [PATCH 2/5] kernel/watchdog_hld: clarify the condition in
 hardlockup_detector_event_create()
Message-Id: <20210914210627.c92374b3726a22014b359dbd@linux-foundation.org>
In-Reply-To: <20210915035103.15586-3-kernelfans@gmail.com>
References: <20210915035103.15586-1-kernelfans@gmail.com>
        <20210915035103.15586-3-kernelfans@gmail.com>
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On Wed, 15 Sep 2021 11:51:00 +0800 Pingfan Liu <kernelfans@gmail.com> wrote:

> hardlockup_detector_event_create() indirectly calls
> kmem_cache_alloc_node(), which is blockable.
> 
> So here, the really planned context is is_percpu_thread().
> 
> ...
>
> --- a/kernel/watchdog_hld.c
> +++ b/kernel/watchdog_hld.c
> @@ -165,10 +165,13 @@ static void watchdog_overflow_callback(struct perf_event *event,
>  
>  static int hardlockup_detector_event_create(void)
>  {
> -	unsigned int cpu = smp_processor_id();
> +	unsigned int cpu;
>  	struct perf_event_attr *wd_attr;
>  	struct perf_event *evt;
>  
> +	/* This function plans to execute in cpu bound kthread */
> +	BUG_ON(!is_percpu_thread());

Can we avoid adding the BUG()?  Find a way to emit a WARNing and then
permit the kernel to continue?

> +	cpu = raw_smp_processor_id();
>  	wd_attr = &wd_hw_attr;
>  	wd_attr->sample_period = hw_nmi_get_sample_period(watchdog_thresh);