[PATCH] printk: fix deadlock when kernel panic

[Muchun Song]

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
        // deal lock
        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 should call printk_safe_flush_on_panic without
holding the read_lock.

Fixes: cf9b1106c81c ("printk/nmi: flush NMI messages on the system panic")
Signed-off-by: Muchun Song <songmuchun@xxxxxxxxxxxxx>
---
 kernel/printk/printk_safe.c | 58 ++++++++++++++++++++++++++-------------------
 1 file changed, 34 insertions(+), 24 deletions(-)

diff --git a/kernel/printk/printk_safe.c b/kernel/printk/printk_safe.c
index a0e6f746de6c..86d9fa74ac5c 100644
--- a/kernel/printk/printk_safe.c
+++ b/kernel/printk/printk_safe.c
@@ -174,30 +174,13 @@ static void report_message_lost(struct printk_safe_seq_buf *s)
 		printk_deferred("Lost %d message(s)!\n", lost);
 }
 
-/*
- * Flush data from the associated per-CPU buffer. The function
- * can be called either via IRQ work or independently.
- */
-static void __printk_safe_flush(struct irq_work *work)
+static void __printk_safe_flush_work(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;
 	size_t len;
-	int i;
+	int i = 0;
 
-	/*
-	 * The lock has two functions. First, one reader has to flush all
-	 * available message to make the lockless synchronization with
-	 * writers easier. Second, we do not want to mix messages from
-	 * different CPUs. This is especially important when printing
-	 * a backtrace.
-	 */
-	raw_spin_lock_irqsave(&read_lock, flags);
-
-	i = 0;
 more:
 	len = atomic_read(&s->len);
 
@@ -232,6 +215,26 @@ static void __printk_safe_flush(struct irq_work *work)
 
 out:
 	report_message_lost(s);
+}
+
+/*
+ * Flush data from the associated per-CPU buffer. The function
+ * can be called either via IRQ work or independently.
+ */
+static void printk_safe_flush_work(struct irq_work *work)
+{
+	unsigned long flags;
+	static DEFINE_RAW_SPINLOCK(read_lock);
+
+	/*
+	 * The lock has two functions. First, one reader has to flush all
+	 * available message to make the lockless synchronization with
+	 * writers easier. Second, we do not want to mix messages from
+	 * different CPUs. This is especially important when printing
+	 * a backtrace.
+	 */
+	raw_spin_lock_irqsave(&read_lock, flags);
+	__printk_safe_flush_work(work);
 	raw_spin_unlock_irqrestore(&read_lock, flags);
 }
 
@@ -248,9 +251,9 @@ void printk_safe_flush(void)
 
 	for_each_possible_cpu(cpu) {
 #ifdef CONFIG_PRINTK_NMI
-		__printk_safe_flush(&per_cpu(nmi_print_seq, cpu).work);
+		printk_safe_flush_work(&per_cpu(nmi_print_seq, cpu).work);
 #endif
-		__printk_safe_flush(&per_cpu(safe_print_seq, cpu).work);
+		printk_safe_flush_work(&per_cpu(safe_print_seq, cpu).work);
 	}
 }
 
@@ -266,6 +269,8 @@ void printk_safe_flush(void)
  */
 void printk_safe_flush_on_panic(void)
 {
+	int cpu;
+
 	/*
 	 * Make sure that we could access the main ring buffer.
 	 * Do not risk a double release when more CPUs are up.
@@ -278,7 +283,12 @@ void printk_safe_flush_on_panic(void)
 		raw_spin_lock_init(&logbuf_lock);
 	}
 
-	printk_safe_flush();
+	for_each_possible_cpu(cpu) {
+#ifdef CONFIG_PRINTK_NMI
+		__printk_safe_flush_work(&per_cpu(nmi_print_seq, cpu).work);
+#endif
+		__printk_safe_flush_work(&per_cpu(safe_print_seq, cpu).work);
+	}
 }
 
 #ifdef CONFIG_PRINTK_NMI
@@ -401,11 +411,11 @@ void __init printk_safe_init(void)
 		struct printk_safe_seq_buf *s;
 
 		s = &per_cpu(safe_print_seq, cpu);
-		init_irq_work(&s->work, __printk_safe_flush);
+		init_irq_work(&s->work, printk_safe_flush_work);
 
 #ifdef CONFIG_PRINTK_NMI
 		s = &per_cpu(nmi_print_seq, cpu);
-		init_irq_work(&s->work, __printk_safe_flush);
+		init_irq_work(&s->work, printk_safe_flush_work);
 #endif
 	}
 
-- 
2.11.0