lockdep是内核提供协助发现死锁问题的功能。
本文首先介绍何为lockdep，然后如何在内核使能lockdep，并简单分析内核lockdep相关代码。
最后构造不同死锁用例，并分析如何根据lockdep输出发现问题根源。
Lockdep介绍
死锁是指两个或多个进程因争夺资源而造成的互相等待的现象。
常见的死锁有如下两种：
递归死锁：中断等延迟操作中使用了锁，和外面的锁构成了递归死锁。
AB-BA死锁：多个锁因处理不当而引发死锁，多个内核路径上的所处理顺序不一致也会导致死锁。
Linux内核提供死锁调试模块Lockdep，跟踪每个锁的自身状态和各个锁之间的依赖关系，经过一系列的验证规则来确保锁之间依赖关系是正确的。
使能Lockdep
Lockdep检测的锁包括spinlock、rwlock、mutex、rwsem的死锁，锁的错误释放，原子操作中睡眠等错误行为。
在内核中配置路径为：Kernel hacking->Lock Debugging (spinlocks, mutexes, etc...)。
下面是lockcep内核选项及其解释：
CONFIG_DEBUG_RT_MUTEXES=y
检测rt mutex的死锁，并自动报告死锁现场信息。
CONFIG_DEBUG_SPINLOCK=y
检测spinlock的未初始化使用等问题。配合NMI watchdog使用，能发现spinlock死锁。
CONFIG_DEBUG_MUTEXES=y
检测并报告mutex错误
CONFIG_DEBUG_WW_MUTEX_SLOWPATH=y
检测wait/wound类型mutex的slowpath测试。
CONFIG_DEBUG_LOCK_ALLOC=y
检测使用中的锁(spinlock/rwlock/mutex/rwsem)被释放，或者使用中的锁被重新初始化，或者在进程退出时持有锁。
CONFIG_PROVE_LOCKING=y
使内核能在死锁发生前报告死锁详细信息。参见/proc/lockdep_chains。
Lock相关内核节点
/proc/sys/kernel/lock_stat------------------------置位则可以查看/proc/lock_stat统计信息，清楚则关闭lockdep统计信息。
/proc/sys/kernel/max_lock_depth--------------
/proc/sys/kernel/prove_locking
/proc/locks
/proc/lock_stat-------------------------------------关于锁的使用统计信息
/proc/lockdep---------------------------------------存在依赖关系的锁
/proc/lockdep_stats------------------------------存在依赖关系锁的统计信息
/proc/lockdep_chains----------------------------依赖关系锁链表
内核还提供了了Tracepoint协助发现锁的使用问题：/sys/kernel/debug/tracing/events/lock。
测试spin_lock死锁
构造测试用例代码如下：
#include <linux/init.h>
#include <linux/module.h>
#include <linux/kernel.h>
#include <linux/spinlock.h>
static DEFINE_SPINLOCK(hack_spinA);
static DEFINE_SPINLOCK(hack_spinB);
void hack_spinAB(void)
{
printk("hack_lockdep: A->B\n");
spin_lock(&hack_spinA);
spin_lock(&hack_spinB);
}
void hack_spinBA(void)
{
printk("hack_lockdep: B->A\n");
spin_lock(&hack_spinB);
}
static int __init lockdep_test_init(void)
{
printk("figo: my lockdep module init");
hack_spinAB();
hack_spinBA();
return 0;
}
static void __exit lockdep_test_exit(void)
{
printk("goodbye\n");
}
MODULE_LICENSE("GPL"); //没有此句不然会造成内核污染，在加载模块时死锁检测机制会被迫关闭：Disabling lock debugging due to kernel taint
module_init(lockdep_test_init);
module_exit(lockdep_test_exit);
执行insmod data/lock.ko 后，(检测发生死锁时报错)控制台显示如下。
首先从死锁描述大概可以知道死锁类型。
然后详细介绍了产生死锁的点，这时就可以大概知道是哪个锁，有哪些地方调用导致了死锁。
接着是详细的发生死锁的backtrace，有助于分析死锁产生时的栈回溯。
figo: my lockdep module init
hack_lockdep:A->B
hack_lockdep:B->A
=============================================
[ INFO: possible recursive locking detected ] -----------------------------------------------检测到的死锁描述：递归死锁类型
4.0.0+ #87 Tainted: G O
---------------------------------------------
insmod/658 is trying to acquire lock: --------------------------------------------------------死锁细节描述：欲持锁点和已持锁点
(hack_spinB){+.+...}, at: [<bf002030>] lockdep_test_init+0x30/0x3c [lock] ------------lockdep_test_init中调用hack_spinBA再次持有hack_spinB锁
but task is already holding lock:
(hack_spinB){+.+...}, at: [<bf000038>] hack_spinAB+0x38/0x3c [lock] -----------------hack_spinB已经在hack_spinAB函数中被持有
other info that might help us debug this: -----------------------------------------------------锁的其它补充信息
Possible unsafe locking scenario:
CPU0
----
lock(hack_spinB);
lock(hack_spinB);
*** DEADLOCK ***
May be due to missing lock nesting notation
2 locks held by insmod/658: -------------------------------------------------------------------进程共持有两个锁
#0: (hack_spinA){+.+...}, at: [<bf000030>] hack_spinAB+0x30/0x3c [lock]
#1: (hack_spinB){+.+...}, at: [<bf000038>] hack_spinAB+0x38/0x3c [lock]
stack backtrace:------------------------------------------栈回溯信息：可以看出从lockdep_test_init->_raw_spin_lock->lock_acquire的调用路径。
CPU: 0 PID: 658 Comm: insmod Tainted: G O 4.0.0+ #87
Hardware name: ARM-Versatile Express
[<c00171b4>] (unwind_backtrace) from [<c0012e7c>] (show_stack+0x20/0x24)
[<c0012e7c>] (show_stack) from [<c05ade10>] (dump_stack+0x8c/0xb4)
[<c05ade10>] (dump_stack) from [<c006b988>] (__lock_acquire+0x1aa4/0x1f64)
[<c006b988>] (__lock_acquire) from [<c006c55c>] (lock_acquire+0xf4/0x190)
[<c006c55c>] (lock_acquire) from [<c05b4ec8>] (_raw_spin_lock+0x60/0x98)
[<c05b4ec8>] (_raw_spin_lock) from [<bf002030>] (lockdep_test_init+0x30/0x3c [lock])
[<bf002030>] (lockdep_test_init [lock]) from [<c0008a28>] (do_one_initcall+0x9c/0x1e8)
[<c0008a28>] (do_one_initcall) from [<c05abf30>] (do_init_module+0x70/0x1c0)
[<c05abf30>] (do_init_module) from [<c00a4ddc>] (load_module+0x18b0/0x1f90)
[<c00a4ddc>] (load_module) from [<c00a55fc>] (SyS_init_module+0x140/0x150)
[<c00a55fc>] (SyS_init_module) from [<c000ec80>] (ret_fast_syscall+0x0/0x4c)
INFO: rcu_sched self-detected stall on CPU
0: (2099 ticks this GP) idle=5ed/140000000000001/0 softirq=13024/13024 fqs=1783
(t=2100 jiffies g=-51 c=-52 q=22)
Task dump for CPU 0:
insmod R running 0 658 657 0x00000002
[<c00171b4>] (unwind_backtrace) from [<c0012e7c>] (show_stack+0x20/0x24)
[<c0012e7c>] (show_stack) from [<c0052874>] (sched_show_task+0x128/0x184)
[<c0052874>] (sched_show_task) from [<c0055dd0>] (dump_cpu_task+0x48/0x4c)
[<c0055dd0>] (dump_cpu_task) from [<c0082878>] (rcu_dump_cpu_stacks+0x9c/0xd4)
[<c0082878>] (rcu_dump_cpu_stacks) from [<c008665c>] (rcu_check_callbacks+0x640/0x968)
[<c008665c>] (rcu_check_callbacks) from [<c008b628>] (update_process_times+0x4c/0x74)
[<c008b628>] (update_process_times) from [<c009a1d4>] (tick_periodic+0x54/0xf8)
[<c009a1d4>] (tick_periodic) from [<c009a3d8>] (tick_handle_periodic+0x38/0x98)
[<c009a3d8>] (tick_handle_periodic) from [<c00164a4>] (twd_handler+0x40/0x50)
[<c00164a4>] (twd_handler) from [<c007dfc4>] (handle_percpu_devid_irq+0xd8/0x1dc)
[<c007dfc4>] (handle_percpu_devid_irq) from [<c0079a7c>] (generic_handle_irq+0x3c/0x4c)
[<c0079a7c>] (generic_handle_irq) from [<c0079dc4>] (__handle_domain_irq+0x6c/0xc4)
[<c0079dc4>] (__handle_domain_irq) from [<c0008740>] (gic_handle_irq+0x34/0x6c)
[<c0008740>] (gic_handle_irq) from [<c0013a44>] (__irq_svc+0x44/0x5c)
Exception stack(0xed5c9d18 to 0xed5c9d60)
9d00: 00000000 00010000
9d20: 0000ffff c02f3898 bf0001b0 c0b1d248 123cc000 00000000 0c99b2c5 00000000
9d40: 00000000 ed5c9d84 ed5c9d60 ed5c9d60 c0070cb4 c0070cb4 60000013 ffffffff
[<c0013a44>] (__irq_svc) from [<c0070cb4>] (do_raw_spin_lock+0xf0/0x1e0)
[<c0070cb4>] (do_raw_spin_lock) from [<c05b4eec>] (_raw_spin_lock+0x84/0x98)
[<c05b4eec>] (_raw_spin_lock) from [<bf002030>] (lockdep_test_init+0x30/0x3c [lock])
[<bf002030>] (lockdep_test_init [lock]) from [<c0008a28>] (do_one_initcall+0x9c/0x1e8)
[<c0008a28>] (do_one_initcall) from [<c05abf30>] (do_init_module+0x70/0x1c0)
[<c05abf30>] (do_init_module) from [<c00a4ddc>] (load_module+0x18b0/0x1f90)
[<c00a4ddc>] (load_module) from [<c00a55fc>] (SyS_init_module+0x140/0x150)
[<c00a55fc>] (SyS_init_module) from [<c000ec80>] (ret_fast_syscall+0x0/0x4c)
BUG: spinlock lockup suspected on CPU#0, insmod/658-----------------------------------------错误类型是spinlock，下面的backtrace和上面基本一致。
lock: hack_spinB+0x0/0xfffffedc [lock], .magic: dead4ead, .owner: insmod/658, .owner_cpu: 0-----------发生死锁的是hack_spinB
CPU: 0 PID: 658 Comm: insmod Tainted: G O 4.0.0+ #87
Hardware name: ARM-Versatile Express
[<c00171b4>] (unwind_backtrace) from [<c0012e7c>] (show_stack+0x20/0x24)
[<c0012e7c>] (show_stack) from [<c05ade10>] (dump_stack+0x8c/0xb4)
[<c05ade10>] (dump_stack) from [<c0070b2c>] (spin_dump+0x8c/0xd0)
[<c0070b2c>] (spin_dump) from [<c0070cd0>] (do_raw_spin_lock+0x10c/0x1e0)
[<c0070cd0>] (do_raw_spin_lock) from [<c05b4eec>] (_raw_spin_lock+0x84/0x98)
[<c05b4eec>] (_raw_spin_lock) from [<bf002030>] (lockdep_test_init+0x30/0x3c [lock])
[<bf002030>] (lockdep_test_init [lock]) from [<c0008a28>] (do_one_initcall+0x9c/0x1e8)
[<c0008a28>] (do_one_initcall) from [<c05abf30>] (do_init_module+0x70/0x1c0)
[<c05abf30>] (do_init_module) from [<c00a4ddc>] (load_module+0x18b0/0x1f90)
[<c00a4ddc>] (load_module) from [<c00a55fc>] (SyS_init_module+0x140/0x150)
[<c00a55fc>] (SyS_init_module) from [<c000ec80>] (ret_fast_syscall+0x0/0x4c)
CONFIG_LOCKDEP=y
整个Lockdep的总开关。参见/proc/lockdep、/proc/lockdep_stats。
CONFIG_LOCK_STAT=y
记锁持有竞争区域的信息，包括等待时间、持有时间等等信息。参见/proc/lock_stat。
CONFIG_DEBUG_LOCKDEP=y
会对Lockdep的使用过程中进行更多的自我检测，会增加很多额外开销。
CONFIG_DEBUG_ATOMIC_SLEEP=y
在atomic section中睡眠可能造成很多不可预测的问题，这些atomic section包括spinlock持锁、rcu读操作、禁止内核抢占部分、中断处理中等等。
