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From:   Pingfan Liu <kernelfans@gmail.com>
To:     linux-kernel@vger.kernel.org
Cc:     Pingfan Liu <kernelfans@gmail.com>, Petr Mladek <pmladek@suse.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Wang Qing <wangqing@vivo.com>,
        "Peter Zijlstra (Intel)" <peterz@infradead.org>,
        Santosh Sivaraj <santosh@fossix.org>,
        linux-arm-kernel@lists.infradead.org
Subject: [PATCHv2 2/4] kernel/watchdog_hld: clarify the condition in hardlockup_detector_event_create()
Date:   Thu, 23 Sep 2021 22:09:49 +0800
Message-Id: <20210923140951.35902-3-kernelfans@gmail.com>
In-Reply-To: <20210923140951.35902-1-kernelfans@gmail.com>
References: <20210923140951.35902-1-kernelfans@gmail.com>
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As for the context, there are two arguments to change
debug_smp_processor_id() to is_percpu_thread().

  -1. watchdog_ev is percpu, and migration will frustrate the attempt
which try to bind a watchdog_ev to a cpu by protecting this func inside
the pair of preempt_disable()/preempt_enable().

  -2. hardlockup_detector_event_create() indirectly calls
kmem_cache_alloc_node(), which is blockable.

So here, spelling out the really planned context "is_percpu_thread()".

Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Cc: Petr Mladek <pmladek@suse.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Wang Qing <wangqing@vivo.com>
Cc: "Peter Zijlstra (Intel)" <peterz@infradead.org>
Cc: Santosh Sivaraj <santosh@fossix.org>
Cc: linux-arm-kernel@lists.infradead.org
To: linux-kernel@vger.kernel.org
---
 kernel/watchdog_hld.c | 5 ++++-
 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/watchdog_hld.c b/kernel/watchdog_hld.c
index 247bf0b1582c..df010df76576 100644
--- 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 */
+	WARN_ON(!is_percpu_thread());
+	cpu = raw_smp_processor_id();
 	wd_attr = &wd_hw_attr;
 	wd_attr->sample_period = hw_nmi_get_sample_period(watchdog_thresh);
 
-- 
2.31.1