[PATCH] rtnetlink: add rtnl_lock debug log

[Rocco Yue <rocco.yue@mediatek.com>]

We often encounter system hangs caused by certain processes
holding rtnl_lock for a long time. Even if there is a lock
detection mechanism in Linux, it is a bit troublesome and
affects the system performance. We hope to add a lightweight
debugging mechanism for detecting rtnl_lock.

Up to now, we have discovered and solved some potential bugs
through such debug information of this lightweight rtnl_lock,
which is helpful for us.

When you say Y for RTNL_LOCK_DEBUG, then the kernel will detect
if any function hold rtnl_lock too long and some key information
will be printed to help identify the issue point.

i.e: from the following logs, we can clear know that the pid=5546
RfxSender_4 process hold rtnl_lock for a long time, causing the
system hang. And we can also speculate that the delay operation
may be performed in devinet_ioctl(), resulting in rtnl_lock was
not released in time.

<6>[  141.151364] ----------- rtnl_print_btrace start -----------
<6>[  141.152079] RfxSender_4[5546][R] hold rtnl_lock more than 2 sec,
start time: 139129481562
<4>[  141.153114]  rtnl_lock+0x88/0xfc
<4>[  141.153523]  devinet_ioctl+0x190/0x1268
<4>[  141.154007]  inet_ioctl+0x108/0x1f4
<4>[  141.154449]  sock_do_ioctl+0x88/0x200
<4>[  141.154911]  sock_ioctl+0x4b0/0x884
<4>[  141.155367]  do_vfs_ioctl+0x6b0/0xcc4
<4>[  141.155830]  __arm64_sys_ioctl+0xc0/0xec
<4>[  141.156326]  el0_svc_common+0x130/0x2c0
<4>[  141.156810]  el0_svc_handler+0xd0/0xe0
<4>[  141.157283]  el0_svc+0x8/0xc
<4>[  141.157646] Call trace:
<4>[  141.157956]  dump_backtrace+0x0/0x240
<4>[  141.158418]  show_stack+0x18/0x24
<4>[  141.158836]  rtnl_print_btrace+0x138/0x1cc
<4>[  141.159362]  call_timer_fn+0x120/0x47c
<4>[  141.159834]  expire_timers+0x28c/0x420
<4>[  141.160306]  __run_timers+0x3d0/0x494
<4>[  141.160768]  run_timer_softirq+0x24/0x48
<4>[  141.161262]  __do_softirq+0x26c/0x968
<4>[  141.161725]  irq_exit+0x1f8/0x2b4
<4>[  141.162145]  __handle_domain_irq+0xdc/0x15c
<4>[  141.162672]  gic_handle_irq+0xe4/0x188
<4>[  141.163144]  el1_irq+0x104/0x200
<4>[  141.163559]  __const_udelay+0x118/0x1b0
<4>[  141.164044]  devinet_ioctl+0x1a0/0x1268
<4>[  141.164527]  inet_ioctl+0x108/0x1f4
<4>[  141.164968]  sock_do_ioctl+0x88/0x200
<4>[  141.165428]  sock_ioctl+0x4b0/0x884
<4>[  141.165868]  do_vfs_ioctl+0x6b0/0xcc4
<4>[  141.166330]  __arm64_sys_ioctl+0xc0/0xec
<4>[  141.166825]  el0_svc_common+0x130/0x2c0
<4>[  141.167308]  el0_svc_handler+0xd0/0xe0
<4>[  141.167786]  el0_svc+0x8/0xc
<6>[  141.168153] ------------ rtnl_print_btrace end -----------

<6>[  147.321389] rtnl_lock is held by [5546] from
[139129481562] to [147321378812]

Signed-off-by: Rocco Yue <rocco.yue@mediatek.com>
---
 lib/Kconfig.debug    |  9 +++++
 net/core/rtnetlink.c | 86 ++++++++++++++++++++++++++++++++++++++++++++
 2 files changed, 95 insertions(+)

diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index 2c7f46b366f1..d0d7457fa394 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -2019,6 +2019,15 @@ config KCOV_IRQ_AREA_SIZE
 	  soft interrupts. This specifies the size of those areas in the
 	  number of unsigned long words.
 
+config RTNL_LOCK_DEBUG
+	bool "rtnl_lock debugging, deadlock detection"
+	select STACKTRACE
+	help
+	  If you say Y here then the kernel will detect whether any process
+	  hold rtnl_lock too long and some key information will be printed
+	  to help identify the issue point.
+	  If unsure, say N.
+
 menuconfig RUNTIME_TESTING_MENU
 	bool "Runtime Testing"
 	def_bool y
diff --git a/net/core/rtnetlink.c b/net/core/rtnetlink.c
index 3485b16a7ff3..ddf374aec6af 100644
--- a/net/core/rtnetlink.c
+++ b/net/core/rtnetlink.c
@@ -57,6 +57,86 @@
 #define RTNL_MAX_TYPE		50
 #define RTNL_SLAVE_MAX_TYPE	40
 
+#ifdef CONFIG_RTNL_LOCK_DEBUG
+#include <linux/sched/debug.h>
+#include <linux/stacktrace.h>
+
+/* Debug log and btrace will be printed when the rtnl_lock
+ * is held for more than RTNL_LOCK_MAX_HOLD_TIME seconds
+ */
+#define RTNL_LOCK_MAX_HOLD_TIME 2
+
+#define RTNL_LOCK_MAX_TRACE     10    /* stack trace length */
+
+struct rtnl_debug_btrace_t {
+	struct task_struct *task;
+	int pid;
+	unsigned long long start_time;
+	unsigned long long end_time;
+	unsigned long   addrs[RTNL_LOCK_MAX_TRACE];
+	unsigned int    nr_entries;
+};
+
+static struct rtnl_debug_btrace_t rtnl_instance = {
+	.task		= NULL,
+	.pid		= 0,
+	.start_time	= 0,
+	.end_time	= 0,
+	.nr_entries	= 0,
+};
+
+static void rtnl_print_btrace(struct timer_list *unused);
+static DEFINE_TIMER(rtnl_chk_timer, rtnl_print_btrace);
+
+/* Save stack trace to the given array of RTNL_LOCK_MAX_TRACE size.
+ */
+static int __save_stack_trace(unsigned long *trace)
+{
+	return stack_trace_save(trace, RTNL_LOCK_MAX_TRACE, 0);
+}
+
+static void rtnl_get_btrace(struct task_struct *who)
+{
+	unsigned long expires;
+
+	rtnl_instance.task = who;
+	rtnl_instance.pid = who->pid;
+	rtnl_instance.start_time = sched_clock();
+	rtnl_instance.end_time = 0;
+	rtnl_instance.nr_entries = __save_stack_trace(rtnl_instance.addrs);
+
+	expires = jiffies + RTNL_LOCK_MAX_HOLD_TIME * HZ;
+	mod_timer(&rtnl_chk_timer, expires);
+}
+
+static void rtnl_print_btrace(struct timer_list *unused)
+{
+	pr_info("----------- %s start -----------\n", __func__);
+	pr_info("%s[%d][%c] hold rtnl_lock more than 2 sec, start time: %llu\n",
+		rtnl_instance.task->comm,
+		rtnl_instance.pid,
+		task_state_to_char(rtnl_instance.task),
+		rtnl_instance.start_time);
+	stack_trace_print(rtnl_instance.addrs, rtnl_instance.nr_entries, 0);
+	show_stack(rtnl_instance.task, NULL, KERN_DEBUG);
+	pr_info("------------ %s end -----------\n", __func__);
+}
+
+static void rtnl_relase_btrace(void)
+{
+	rtnl_instance.end_time = sched_clock();
+
+	if (rtnl_instance.end_time - rtnl_instance.start_time > 2000000000ULL) {
+		pr_info("rtnl_lock is held by [%d] from [%llu] to [%llu]\n",
+			rtnl_instance.pid,
+			rtnl_instance.start_time,
+			rtnl_instance.end_time);
+	}
+
+	del_timer(&rtnl_chk_timer);
+}
+#endif
+
 struct rtnl_link {
 	rtnl_doit_func		doit;
 	rtnl_dumpit_func	dumpit;
@@ -70,6 +150,9 @@ static DEFINE_MUTEX(rtnl_mutex);
 void rtnl_lock(void)
 {
 	mutex_lock(&rtnl_mutex);
+#ifdef CONFIG_RTNL_LOCK_DEBUG
+	rtnl_get_btrace(current);
+#endif
 }
 EXPORT_SYMBOL(rtnl_lock);
 
@@ -104,6 +187,9 @@ void __rtnl_unlock(void)
 		cond_resched();
 		head = next;
 	}
+#ifdef CONFIG_RTNL_LOCK_DEBUG
+	rtnl_relase_btrace();
+#endif
 }
 
 void rtnl_unlock(void)
-- 
2.18.0