Received-SPF: pass (google.com: domain of linux-kernel-owner@vger.kernel.org designates 23.128.96.18 as permitted sender) client-ip=23.128.96.18;
From:   Muchun Song <songmuchun@bytedance.com>
To:     pmladek@suse.com, sergey.senozhatsky@gmail.com,
        rostedt@goodmis.org, john.ogness@linutronix.de,
        akpm@linux-foundation.org
Cc:     linux-kernel@vger.kernel.org,
        Muchun Song <songmuchun@bytedance.com>
Subject: [PATCH v2] printk: fix deadlock when kernel panic
Date:   Sat,  6 Feb 2021 13:41:24 +0800
Message-Id: <20210206054124.6743-1-songmuchun@bytedance.com>
MIME-Version: 1.0
Content-Transfer-Encoding: 8bit
Precedence: bulk

We found a deadlock bug on our server when the kernel panic. It can be
described in the following diagram.

CPU0:                                         CPU1:
panic                                         rcu_dump_cpu_stacks
  kdump_nmi_shootdown_cpus                      nmi_trigger_cpumask_backtrace
    register_nmi_handler(crash_nmi_callback)      printk_safe_flush
                                                    __printk_safe_flush
                                                      raw_spin_lock_irqsave(&read_lock)
    // send NMI to other processors
    apic_send_IPI_allbutself(NMI_VECTOR)
                                                        // NMI interrupt, dead loop
                                                        crash_nmi_callback
  printk_safe_flush_on_panic
    printk_safe_flush
      __printk_safe_flush
        // deadlock
        raw_spin_lock_irqsave(&read_lock)

The register_nmi_handler() can be called in the __crash_kexec() or the
crash_smp_send_stop() on the x86-64. Because CPU1 is interrupted by the
NMI with holding the read_lock and crash_nmi_callback() never returns,
CPU0 can deadlock when printk_safe_flush_on_panic() is called.

When we hold the read_lock and then interrupted by the NMI, if the NMI
handler call nmi_panic(), it is also can lead to deadlock.

In order to fix it, we make read_lock global and rename it to
safe_read_lock. And we handle safe_read_lock the same way in
printk_safe_flush_on_panic() as we handle logbuf_lock there.

Fixes: cf9b1106c81c ("printk/nmi: flush NMI messages on the system panic")
Signed-off-by: Muchun Song <songmuchun@bytedance.com>
---
v2:
 - handle read_lock the same way as we handle logbuf_lock there.

 Thanks Petr.

 kernel/printk/printk_safe.c | 16 ++++++++++++----
 1 file changed, 12 insertions(+), 4 deletions(-)

diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index a0e6f746de6c..2e9e3ed7d63e 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -45,6 +45,8 @@ struct printk_safe_seq_buf {
 static DEFINE_PER_CPU(struct printk_safe_seq_buf, safe_print_seq);
 static DEFINE_PER_CPU(int, printk_context);
 
+static DEFINE_RAW_SPINLOCK(safe_read_lock);
+
 #ifdef CONFIG_PRINTK_NMI
 static DEFINE_PER_CPU(struct printk_safe_seq_buf, nmi_print_seq);
 #endif
@@ -180,8 +182,6 @@ static void report_message_lost(struct printk_safe_seq_buf *s)
  */
 static void __printk_safe_flush(struct irq_work *work)
 {
-	static raw_spinlock_t read_lock =
-		__RAW_SPIN_LOCK_INITIALIZER(read_lock);
 	struct printk_safe_seq_buf *s =
 		container_of(work, struct printk_safe_seq_buf, work);
 	unsigned long flags;
@@ -195,7 +195,7 @@ static void __printk_safe_flush(struct irq_work *work)
 	 * different CPUs. This is especially important when printing
 	 * a backtrace.
 	 */
-	raw_spin_lock_irqsave(&read_lock, flags);
+	raw_spin_lock_irqsave(&safe_read_lock, flags);
 
 	i = 0;
 more:
@@ -232,7 +232,7 @@ static void __printk_safe_flush(struct irq_work *work)
 
 out:
 	report_message_lost(s);
-	raw_spin_unlock_irqrestore(&read_lock, flags);
+	raw_spin_unlock_irqrestore(&safe_read_lock, flags);
 }
 
 /**
@@ -278,6 +278,14 @@ void printk_safe_flush_on_panic(void)
 		raw_spin_lock_init(&logbuf_lock);
 	}
 
+	if (raw_spin_is_locked(&safe_read_lock)) {
+		if (num_online_cpus() > 1)
+			return;
+
+		debug_locks_off();
+		raw_spin_lock_init(&safe_read_lock);
+	}
+
 	printk_safe_flush();
 }
 
-- 
2.11.0