lockdep是内核引入的死锁检测机制，只需开启这一特性，lockdep会自动在锁初始化时记录这个锁的类型（每个lock代码初始化位置对应一个锁类，key），在申请和释放时会更新内部锁关系数据链表，通过内部的死锁检测算法，在发生死锁时打印出死锁原因。
关注这一特性，不需要过多关注内部算法实现，更多时候，我们需要知道如何使用他，如何阅读lockdep信息。
- /include/linux/lockdep.h
- /kernel/locking/lockdep.c
初始化
spin_lock_init()
↓
raw_spin_lock_init()
↓
__raw_spin_lock_init()
→ debug_check_no_locks_freed()
→ lockdep_init_map()
→ 初始化 spin_lock 的值
# define raw_spin_lock_init(lock) \
do { \
static struct lock_class_key __key; \
__raw_spin_lock_init((lock), #lock, &__key); \
} while (0)
锁申请
spin_lock()
↓
raw_spin_lock()
↓
_raw_spin_lock() @kernel/spinlock.c
↓
__raw_spin_lock() @include/linux/spinlock_api_smp.h
→ preempt_disable();
→ spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);
↓
lock_acquire() → __lock_acquire() → __lock_acquire()
__lock_acquire() 是 lockdep 死锁检测的核心，所有原理中描述的死锁错误都是在这里检测的。如果出错，最终会调用 print_xxx_bug() 函数。
→ LOCK_CONTENDED(lock, do_raw_spin_trylock, do_raw_spin_lock);
Enable lockdep with menu config
[*] Detect Hard and Soft Lockups
[*] Detect Hung Tasks
[*] RT Mutex debugging, deadlock detection
-*- Spinlock and rw-lock debugging: basic checks
-*- Mutex debugging: basic checks
-*- Lock debugging: detect incorrect freeing of live locks
[*] Lock debugging: prove locking correctness
[*] Lock usage statistics
Enable lockdep with defconfig
// This feature enables the kernel to report locking related deadlocks before they actually occur. For more details, see Documentation/locking/lockdep-design.txt.
CONFIG_PROVE_LOCKING=y
// Detect incorrect freeing of live locks.
CONFIG_DEBUG_LOCK_ALLOC=y
// The lock dependency engine will do additional runtime checks to debug itself, at the price of more runtime overhead.
CONFIG_DEBUG_LOCKDEP=y
// Lock usage statistics. For more details, see Documentation/locking/lockstat.txt
CONFIG_LOCK_STAT=y
// The kernel to run a short self-test during bootup in start_kernel(). The self-test checks whether common types of locking bugs are detected by debugging mechanisms or not. For more details, see lib/locking-selftest.c
CONFIG_DEBUG_LOCKING_API_SELFTESTS=y
CONFIG_LOCKUP_DETECTOR=y
CONFIG_HARDLOCKUP_DETECTOR_OTHER_CPU=y
CONFIG_HARDLOCKUP_DETECTOR=y
CONFIG_DEBUG_ATOMIC_SLEEP=y
Check lockdep info
/proc/lockdep
/proc/lockdep_chains
/proc/lockdep_stat
/proc/locks
/proc/lock_stats
Check lockdep warnings
如果发生deadlock，会打出warning和典型场景图帮助分析。
Lock status
https://www.kernel.org/doc/html/latest/locking/lockdep-design.html#state
一个锁类的使用历史，可以归纳到 4n + 1 种使用情况：
- 'ever held in [hardirq or softirq] context' （2）
- 'ever held as readlock in [hardirq or softirq] context' （2）
- 'ever held with [hardirq or softirq] enabled' （2）
- 'ever held as readlock with [hardirq or softirq] enabled' （2）
- 'ever used' [ == !unused ] （1）
比如：
(&sio_locks[i].lock){-.-.}, at: [<c02867fd>] mutex_lock+0x21/0x24
||||
||| \-> softirq disabled and not in softirq context (softirq read lock)
|| \--> ever acquired in softirq context (softirq)
| \---> hardirq disabled and not in hardirq context (hardirq read lock)
\----> ever acquired in hardirq context (hardirq)
大括号反映了这个锁的持锁历史，每个bit分别代表：hardirq，hardirq-read，softirq，softirq-read
各个字符代表的含义分别如下：
‘.’ acquired while irqs disabled and not in irq context
‘-’ acquired in irq context
‘+’ acquired with irqs enabled
‘?’ acquired in irq context with irqs enabled.
或者用矩阵表示：
irq enabled irq disabled
ever in irq ‘?’ ‘-’
never in irq ‘+’ ‘.’
上面样例中，两个“-”代表在softirq、hardirq都有出现过，并且当时是irq disable的状态。（不确定理解是否正确。）
Deplock checking rules
单锁检测规则：
一个锁，一定不可能有的地方用lock_irqsave（关中断），有的地方用lock（不关中断），见locking/spinlock，那么一旦发现这类冲突，则会报出单锁检测的ERROR。
下面四个定义，正好对应了四个锁函数。
多锁依赖规则：
就是锁间依赖关系的检测算法
Typical warnings
[ INFO: possible circular locking dependency detected ] // 圆形锁，获取锁的顺序异常（ABBA）
[ INFO: %s-safe -> %s-unsafe lock order detected ] // 获取从 safe 的锁类到 unsafe 的锁类的操作
[ INFO: possible recursive locking detected ] // 重复去获取同类锁（AA）
[ INFO: inconsistent lock state ] // 锁的状态前后不一致
[ INFO: possible irq lock inversion dependency detected ] // 嵌套获取锁的状态前后需要保持一致，即 [hardirq-safe] -> [hardirq-unsafe]，[softirq-safe] -> [softirq-unsafe] 会警报死锁风险
[ INFO: suspicious RCU usage. ] // 可疑的 RCU 用法
https://www.kernel.org/doc/html/latest/locking/lockdep-design.html
https://lwn.net/Articles/185666/