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Date:   Wed, 22 Nov 2023 14:05:21 -0800
From:   Andrew Morton <akpm@linux-foundation.org>
To:     lizhe.67@bytedance.com
Cc:     dianders@chromium.org, pmladek@suse.com,
        lecopzer.chen@mediatek.com, kernelfans@gmail.com,
        linux-kernel@vger.kernel.org, lizefan.x@bytedance.com
Subject: Re: [PATCH v2] softlockup: serialized softlockup's log
Message-Id: <20231122140521.85c66b789625e8d270722b3c@linux-foundation.org>
In-Reply-To: <20231122100212.94327-1-lizhe.67@bytedance.com>
References: <20231122100212.94327-1-lizhe.67@bytedance.com>
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On Wed, 22 Nov 2023 18:02:12 +0800 lizhe.67@bytedance.com wrote:

> From: Li Zhe <lizhe.67@bytedance.com>
> 
> If multiple CPUs trigger softlockup at the same time with
> 'softlockup_all_cpu_backtrace=0', the softlockup's logs will appear
> staggeredly in dmesg, which will affect the viewing of the logs for
> developer. Since the code path for outputting softlockup logs is not
> a kernel hotspot and the performance requirements for the code are
> not strict, locks are used to serialize the softlockup log output to
> improve the readability of the logs.

Seems reasonable, but...

> --- a/kernel/watchdog.c
> +++ b/kernel/watchdog.c
> @@ -28,6 +28,8 @@
>  #include <linux/kvm_para.h>
>  
>  static DEFINE_MUTEX(watchdog_mutex);
> +/* This lock is used to prevent concurrent actions of softlockup output logs */
> +static DEFINE_SPINLOCK(watchdog_output_lock);

It would be a little neater to reduce the scope of this - move the
definition into that little code block in watchdog_timer_fn() where it
is actually used.

>  #if defined(CONFIG_HARDLOCKUP_DETECTOR) || defined(CONFIG_HARDLOCKUP_DETECTOR_SPARC64)
>  # define WATCHDOG_HARDLOCKUP_DEFAULT	1
> @@ -514,6 +516,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
>  		/* Start period for the next softlockup warning. */
>  		update_report_ts();
>  
> +		spin_lock(&watchdog_output_lock);

The hrtimer callout function is called from [soft]irq context, yes? 
Doesn't lockdep get upset when we take a spinlock in such a context?

>  		pr_emerg("BUG: soft lockup - CPU#%d stuck for %us! [%s:%d]\n",
>  			smp_processor_id(), duration,
>  			current->comm, task_pid_nr(current));
> @@ -523,6 +526,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
>  			show_regs(regs);
>  		else
>  			dump_stack();
> +		spin_unlock(&watchdog_output_lock);
>  
>  		if (softlockup_all_cpu_backtrace) {
>  			trigger_allbutcpu_cpu_backtrace(smp_processor_id());