Linux 内核 spin_lock 和spin_unlock

本文详细阐述了spin_lock在ARMv8架构中的具体实现方式,重点介绍了spin_lock和spin_unlock的功能及内部机制,通过源码分析揭示了其在内核中的高效同步原理。

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1. spin_lock作为一种轻量级的同步机制,在内核应用广泛,本文将基于ARMv8讲述其具体实现。由于spin_lock家族比较庞大,不大算面面俱到的介绍每一个实现,重点介绍spin_lock和spin_unlock的实现。

include/linux/spinlock.h


static inline void spin_lock(spinlock_t *lock)
{
        raw_spin_lock(&lock->rlock);
}

static inline void spin_unlock(spinlock_t *lock)
{
        raw_spin_unlock(&lock->rlock);
}

#define raw_spin_lock(lock)     _raw_spin_lock(lock)

#define raw_spin_unlock(lock)           _raw_spin_unlock(lock)


include/linux/spinlock_api_smp.h


void __lockfunc _raw_spin_lock(raw_spinlock_t *lock)            __acquires(lock);

void __lockfunc _raw_spin_unlock(raw_spinlock_t *lock)          __releases(lock);


kernel/spinlock.c


void __lockfunc _raw_spin_lock(raw_spinlock_t *lock)
{
        __raw_spin_lock(lock);
}
EXPORT_SYMBOL(_raw_spin_lock);

void __lockfunc _raw_spin_unlock(raw_spinlock_t *lock)
{
        __raw_spin_unlock(lock);
}
EXPORT_SYMBOL(_raw_spin_unlock);


include/linux/spinlock_api_smp.h


static inline void __raw_spin_lock(raw_spinlock_t *lock)
{
        preempt_disable();                          //禁止强占
        spin_acquire(&lock->dep_map, 0, 0, _RET_IP_);
        LOCK_CONTENDED(lock, do_raw_spin_trylock, do_raw_spin_lock);
}

static inline void __raw_spin_unlock(raw_spinlock_t *lock)
{
        spin_release(&lock->dep_map, 1, _RET_IP_);
        do_raw_spin_unlock(lock);
        preempt_enable();
}


include/linux/lockdep.h


#define LOCK_CONTENDED(_lock, try, lock)                        \
do {                                                            \
        if (!try(_lock)) {                                      \
                lock_contended(&(_lock)->dep_map, _RET_IP_);    \
                lock(_lock);                                    \
        }                                                       \
        lock_acquired(&(_lock)->dep_map, _RET_IP_);                     \
} while (0)

由以上函数调用可以看出spin_lock调用do_raw_spin_lock, spin_unlock调用do_raw_spin_unlock。


include/linux/spinlock.h


static inline void do_raw_spin_lock(raw_spinlock_t *lock) __acquires(lock)
{
        __acquire(lock);
        arch_spin_lock(&lock->raw_lock);
}
static inline void do_raw_spin_unlock(raw_spinlock_t *lock) __releases(lock)
{
        arch_spin_unlock(&lock->raw_lock);
        __release(lock);
}


2. Armv8 arch_spin_lock 和 arch_spin_unlock


 41 static inline void arch_spin_lock(arch_spinlock_t *lock)
 42 {
 43         unsigned int tmp;
 44
 45         asm volatile(
 46         "       sevl\n"
 47         "1:     wfe\n"
 48         "2:     ldaxr   %w0, %1\n"
 49         "       cbnz    %w0, 1b\n"
 50         "       stxr    %w0, %w2, %1\n"
 51         "       cbnz    %w0, 2b\n"
 52         : "=&r" (tmp), "+Q" (lock->lock)
 53         : "r" (1)
 54         : "cc", "memory");
 55 }

46行,设置本地事件

47行, 等待事件(可能进入低功耗状态)

48行,独占读取lock->lock

49行, 如果不是0,跳转到47行,等待事件。如果是,继续执行。

50行, 独占向lock->lock写入1.

51行, 如果不成功,跳转到48行。如果成功结束。


 73 static inline void arch_spin_unlock(arch_spinlock_t *lock)
 74 {
 75         asm volatile(
 76         "       stlr    %w1, %0\n"
 77         : "=Q" (lock->lock) : "r" (0) : "memory");
 78 }

76行,向lock->lock写入0;会发出唤醒事件。





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