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From:   lizhe.67@bytedance.com
To:     dianders@chromium.org, akpm@linux-foundation.org, pmladek@suse.com,
        lecopzer.chen@mediatek.com, kernelfans@gmail.com
Cc:     linux-kernel@vger.kernel.org, lizefan.x@bytedance.com,
        lizhe.67@bytedance.com
Subject: [PATCH v3] softlockup: serialized softlockup's log
Date:   Thu, 23 Nov 2023 16:40:22 +0800
Message-Id: <20231123084022.10302-1-lizhe.67@bytedance.com>
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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.

Signed-off-by: Li Zhe <lizhe.67@bytedance.com>
---
Changelogs:

v1->v2:
- define the lock outside the scope of function
- add precondition 'softlockup_all_cpu_backtrace=0' in commit message

v2->v3:
- define the lock inside the scope of function

 kernel/watchdog.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/kernel/watchdog.c b/kernel/watchdog.c
index 5cd6d4e26915..bf30a6fac665 100644
--- a/kernel/watchdog.c
+++ b/kernel/watchdog.c
@@ -448,6 +448,7 @@ static enum hrtimer_restart watchdog_timer_fn(struct hrtimer *hrtimer)
 	struct pt_regs *regs = get_irq_regs();
 	int duration;
 	int softlockup_all_cpu_backtrace = sysctl_softlockup_all_cpu_backtrace;
+	static DEFINE_SPINLOCK(watchdog_output_lock);
 
 	if (!watchdog_enabled)
 		return HRTIMER_NORESTART;
@@ -514,6 +515,7 @@ 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);
 		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 +525,7 @@ 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());
-- 
2.20.1