Linux 4.19 内核中 spinlock 概览

Linux内核中 spinlock相关数据结构和代码实现涉及的文件还是挺多的，这篇博客尝试从文件的角度来梳理一下 spinlock的相关数据结构和代码实现，适合想大概了解 Linux内核中 spinlock从上层 API到底层实现间的调用路径和传参变化，尤其适合了解 spinlock和 qspinlock之间的关系。

这里只以 spinlock的 加锁函数 为切入点，梳理该函数从上层 API到底层实现涉及的文件。

1. 关注 spin_lock()

spinlock的加锁函数是 spin_lock()
spin_lock()的位置：include/linux/spinlock.h
spin_lock()的内容：

static __always_inline void spin_lock(spinlock_t *lock)
{
    raw_spin_lock(&lock->rlock);
}
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看到这里，需要关注该函数的入参类型：spinlock_t
spinlock_t的位置： include/linux/spinlock_types.h
spinlock_t的内容：

typedef struct spinlock {
    union {
        struct raw_spinlock rlock;

#ifdef CONFIG_DEBUG_LOCK_ALLOC
# define LOCK_PADSIZE (offsetof(struct raw_spinlock, dep_map))
        struct {
            u8 __padding[LOCK_PADSIZE];
            struct lockdep_map dep_map;
        };
#endif
    };
} spinlock_t;
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2. 关注 __raw_spin_lock()

从上面可以看到，spin_lock()调用了 raw_spin_lock(), 而 raw_spin_lock()是一个宏，将它展开后得到：__raw_spin_lock()，同时假定 CONFIG_INLINE_SPIN_LOCK配置项打开，则
__raw_spin_lock()的位置：include/linux/spinlock_api_smp.h
__raw_spin_lock()的内容：

static inline void __raw_spin_lock(raw_spinlock_t *lock)                                       
{
    preempt_disable();
    spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);
    LOCK_CONTENDED(lock, do_raw_spin_trylock, do_raw_spin_lock);
}
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__raw_spin_lock()的参数类型是：raw_spinlock_t，它和 struct raw_spinlock是同一个类型，对应 spinlock_t中 rlock成员变量的类型
raw_spinlock_t的位置：include/linux/spinlock_types.h
raw_spinloct_t的内容：

typedef struct raw_spinlock {
    arch_spinlock_t raw_lock;
#ifdef CONFIG_DEBUG_SPINLOCK          
    unsigned int magic, owner_cpu;
    void *owner;                
#endif                                
#ifdef CONFIG_DEBUG_LOCK_ALLOC        
    struct lockdep_map dep_map;
#endif                                
} raw_spinlock_t;
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3. 关注 do_raw_spin_lock()

从上面可以看到， __raw_spin_lock()调用了 do_raw_spin_lock()
do_raw_spin_lock()的位置：include/linux/spinlock.h
do_raw_spin_lock()的内容：

static inline void do_raw_spin_lock(raw_spinlock_t *lock) __acquires(lock)
{
    __acquire(lock);
    arch_spin_lock(&lock->raw_lock);
}
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4. 关注 arch_spin_lock() 和 arch_spinlock_t

从上面可以看到，do_raw_spin_lock()调用了 arch_spin_lock()
arch_spin_lock()和它的入参 arch_spinlock_t是与体系结构相关的，这里面基本能分成两派，
一派是自己实现 arch_spin_lock()和 arch_spinlock_t，
一派是使用通用的 qspinlock实现，也即 #define arch_spin_lock(l) queued_spin_lock(l)和 typedef struct qspinlock {...} arch_spinlock_t

体系结构自己实现的架构包括：alpha, arc, arm, hexagon, ia64, parisc, powerpc, riscv, s390, sh, sparc-32, xtensa
具体列举其中的 5 个：
alpha架构
arch_spin_lock()的位置：arch/alpha/include/asm/spinlock.h
arch_spinlock_t的位置：arch/alpha/include/asm/spinlock_types.h

arc架构
arch_spin_lock()的位置：arch/arc/include/asm/spinlock.h
arch_spinlock_t的位置：arch/arc/include/asm/spinlock_types.h

arm架构
arch_spin_lock()的位置：arch/arm/include/asm/spinlock.h
arch_spinlock_t的位置：arch/arm/include/asm/spinlock_types.h

hexagon架构
arch_spin_lock()的位置：arch/hexagon/include/asm/spinlock.h
arch_spinlock_t的位置：arch/hexagon/include/asm/spinlock_types.h

ia64架构
arch_spin_lock()的位置：arch/ia64/include/asm/spinlock.h
arch_spinlock_t的位置：arch/ia64/include/asm/spinlock_types.h

使用 qspinlock实现的架构包括：mips, sparc-64, x86, arm64, openrisc
queued_spin_lock()的位置：include/asm-generic/qspinlock.h
queued_spin_lock()的内容是：

static __always_inline void queued_spin_lock(struct qspinlock *lock)
{
    u32 val;
 
    val = atomic_cmpxchg_acquire(&lock->val, 0, _Q_LOCKED_VAL);
    if (likely(val == 0)) 
        return;
    queued_spin_lock_slowpath(lock, val);
}
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struct qspinlock的位置：include/asm-generic/qspinlock_types.h
struct qspinlock的内容是：

typedef struct qspinlock {
    union {
        atomic_t val;

        /*
         * By using the whole 2nd least significant byte for the
         * pending bit, we can allow better optimization of the lock
         * acquisition for the pending bit holder.
         */
#ifdef __LITTLE_ENDIAN
        struct {
            u8  locked;
            u8  pending;
        };
        struct {
            u16 locked_pending;
            u16 tail;
        };
#else
        struct {
            u16 tail;
            u16 locked_pending;
        };
        struct {
            u8  reserved[2];
            u8  pending;
            u8  locked;
        };
#endif
    };
} arch_spinlock_t;
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