在之前的一篇博客中,着重讲解了在Linux内核中同步方法--对于整型的原子操作,除此之外,内核同步方法中还有对位的原子操作.下面我们来列举一下原子位操作的列表:
原子位操作 | 描述 |
---|---|
void set_bit(int nr, volatile unsigned long *addr) | 原子的设置addr所指对象的第nr位 |
void clear_bit(int nr, volatile unsigned long *addr) | 原子的清空addr所指对象的第nr位 |
void change_bit(int nr, volatile unsigned long *addr) | 原子的翻转addr所指对象的第nr位 |
int test_and_set_bit(int nr, volatile unsigned long *addr) | 原子的设置addr所指对象的第nr位,并返回原先的值 |
int test_and_clear_bit(int nr, volatile unsigned long *addr) | 原子的清空addr所指对象的第nr位,并返回原先的值 |
int test_and_change_bit(int nr, volatile unsigned long *addr) | 原子的翻转addr所指对象的第nr位,并返回原先的值 |
好了,现在有了一个整体的了解之后,我就可以附上代码了,主要的解释都在代码的注释中.
#ifndef _ASM_GENERIC_BITOPS_ATOMIC_H_
#define _ASM_GENERIC_BITOPS_ATOMIC_H_
#include <asm/types.h>
#include <asm/system.h>
#ifdef CONFIG_SMP
#include <asm/spinlock.h>
#include <asm/cache.h> /* we use L1_CACHE_BYTES 我们使用L1_CACHE_BYTES */
/* * 我们将下面用到的几个宏放在这里. * #define L1_CACHE_SHIFT 5 * #define L1_CACHE_BYTES (1 << L1_CACHE_SHIFT) * //BITS_PER_LONG 32 * #define BIT(nr) (1UL << (nr)) * #define BIT_MASK(nr) (1UL << ((nr) % BITS_PER_LONG)) * #define BIT_WORD(nr) ((nr) / BITS_PER_LONG) * #define BITS_PER_BYTE 8 * #define BITS_TO_LONGS(nr) DIV_ROUND_UP(nr, BITS_PER_BYTE * sizeof(long)) * */
/* Use an array of spinlocks for our atomic_ts. * Hash function to index into a different SPINLOCK. * Since "a" is usually an address, use one spinlock per cacheline. * * 为我们的atomic_ts使用一个spinlocks的数组. * 使用哈希函数来索引到一个不同的SPINLOCK * "a"通常是一个地址,每一个缓存行使用一个spinlock */
# define ATOMIC_HASH_SIZE 4
# define ATOMIC_HASH(a) (&(__atomic_hash[ (((unsigned long) a)/L1_CACHE_BYTES) & (ATOMIC_HASH_SIZE-1) ]))
extern arch_spinlock_t __atomic_hash[ATOMIC_HASH_SIZE] __lock_aligned;
/* Can't use raw_spin_lock_irq because of #include problems, so * this is the substitute * 不能使用raw_spin_lock_irq因为会出现#include 问题,所以 * 下面这个是他的替代品. */
#define _atomic_spin_lock_irqsave(l,f) do { \
arch_spinlock_t *s = ATOMIC_HASH(l); \
local_irq_save(f); \
arch_spin_lock(s); \
} while(0)
#define _atomic_spin_unlock_irqrestore(l,f) do { \
arch_spinlock_t *s = ATOMIC_HASH(l); \
arch_spin_unlock(s); unsigned \
local_irq_restore(f); \
} while(0)
#else
# define _atomic_spin_lock_irqsave(l,f) do { local_irq_save(f); } while (0)
# define _atomic_spin_unlock_irqrestore(l,f) do { local_irq_restore(f); } while (0)
#endif
/* * NMI events can occur at any time, including when interrupts have been * disabled by *_irqsave(). So you can get NMI events occurring while a * *_bit function is holding a spin lock. If the NMI handler also wants * to do bit manipulation (and they do) then you can get a deadlock * between the original caller of *_bit() and the NMI handler. * * by Keith Owens * * NMI事件随时是会发生的,包括当中断已经被 *_irqsave()禁止了.所以,当一个 *_bit * 函数正在使用一个spin锁的时候,NMI事件也可能会发生.如果NMI处理函数也想要进行 * 位操作,那么在原来的 *_bit()调用者和NMI处理函数之间就会产生死锁. */
/** * set_bit - Atomically set a bit in memory * @nr: the bit to set * @addr: the address to start counting from * set_bit - 原子的在内存中设置一位 * @nr: 要被设置的那一位 * @addr: 开始计数的地址 * * This function is atomic and may not be reordered. See __set_bit() * if you do not require the atomic guarantees. * 这个函数是原子的,并且不能被重新排序.如果你不需要原子保证,可以 * 看一下 __set_bit()这个函数. * * Note: there are no guarantees that this function will not be reordered * on non x86 architectures, so if you are writing portable code, * make sure not to rely on its reordering guarantees. * 注意:在非x86架构上,对这个函数不能被重新排序是不能被保证的.所以,如果你正在 * 便携式的代码,一定要确定不能依赖于他的重新排序保障. * * Note that @nr may be almost arbitrarily large; this function is not * restricted to acting on a single-word quantity. * * 注意:nr可能是任意大的;这个函数不是被限制在一个单字的量上的. */
static inline void set_bit(int nr, volatile unsigned long *addr)
{
unsigned long mask = BIT_MASK(nr);
unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
unsigned long flags;
_atomic_spin_lock_irqsave(p, flags);
//这个地方基本上是关于位的操作,相对来说比较简单,就是移位
*p |= mask;
_atomic_spin_unlock_irqrestore(p, flags);
}
/** * clear_bit - Clears a bit in memory * @nr: Bit to clear * @addr: Address to start counting from * * clear_bit - 清除内存中的一个位 * @nr: 要被清楚的那个位 * @addr: 开始计数的位置 * * clear_bit() is atomic and may not be reordered. However, it does * not contain a memory barrier, so if it is used for locking purposes, * you should call smp_mb__before_clear_bit() and/or smp_mb__after_clear_bit() * in order to ensure changes are visible on other processors. * * clear_bit()是原子的并且不能被重新换顺序.然而,他并不包含一个内存屏障,所以, * 如果他被用来实现锁的目的,你应该调用smp_mb_before_clear_bit()或者是smp_mb_after_clear_bit() * 来确保在其他处理器上改变是可见的. */
static inline void clear_bit(int nr, volatile unsigned long *addr)
{
unsigned long mask = BIT_MASK(nr);
unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
unsigned long flags;
_atomic_spin_lock_irqsave(p, flags);
*p &= ~mask;
_atomic_spin_unlock_irqrestore(p, flags);
}
/** * change_bit - Toggle a bit in memory * @nr: Bit to change * @addr: Address to start counting from * change_bit - 在内存中切换一个位 * @nr: 要被改变的位 * @addr: 开始计数的地址 * * change_bit() is atomic and may not be reordered. It may be * reordered on other architectures than x86. * Note that @nr may be almost arbitrarily large; this function is not * restricted to acting on a single-word quantity. * 同上. */
static inline void change_bit(int nr, volatile unsigned long *addr)
{
unsigned long mask = BIT_MASK(nr);
unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
unsigned long flags;
_atomic_spin_lock_irqsave(p, flags);
*p ^= mask;
_atomic_spin_unlock_irqrestore(p, flags);
}
/** * test_and_set_bit - Set a bit and return its old value * @nr: Bit to set * @addr: Address to count from * test_and_set_bit - 设置一个位并且返回他的原来的值 * @nr: 被设置的位 * @addr: 开始计数的地址 * * This operation is atomic and cannot be reordered. * It may be reordered on other architectures than x86. * It also implies a memory barrier. * 该操作是原子的并且不能被重新排序.他可能在其他架构中 * 被重新排序而不是在x86架构上.他也包含了一个内存屏障. */
static inline int test_and_set_bit(int nr, volatile unsigned long *addr)
{
unsigned long mask = BIT_MASK(nr);
unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
unsigned long old;
unsigned long flags;
_atomic_spin_lock_irqsave(p, flags);
old = *p;
*p = old | mask;
_atomic_spin_unlock_irqrestore(p, flags);
return (old & mask) != 0;
}
/** * test_and_clear_bit - Clear a bit and return its old value * @nr: Bit to clear * @addr: Address to count from * test_and_clear_bit - 清楚一个位并且返回他原来的值 * @nr: 被清除的位 * @addr: 开始计数的地址 * * This operation is atomic and cannot be reordered. * It can be reorderdered on other architectures other than x86. * It also implies a memory barrier. * 该操作是原子的并且不能被重新排序.他可能在其他架构中 * 被重新排序而不是在x86架构上.他也包含了一个内存屏障. */
static inline int test_and_clear_bit(int nr, volatile unsigned long *addr)
{
unsigned long mask = BIT_MASK(nr);
unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
unsigned long old;
unsigned long flags;
_atomic_spin_lock_irqsave(p, flags);
old = *p;
*p = old & ~mask;
_atomic_spin_unlock_irqrestore(p, flags);
return (old & mask) != 0;
}
/** * test_and_change_bit - Change a bit and return its old value * @nr: Bit to change * @addr: Address to count from * test_and_change_bit - 改变一个位并且返回他原来的值 * @nr: 被改变的值 * @addr: 开始计数的位置 * * This operation is atomic and cannot be reordered. * It also implies a memory barrier. * 该操作是原子的并且不能被重新排序.他也包含了一个内存屏障. */
static inline int test_and_change_bit(int nr, volatile unsigned long *addr)
{
unsigned long mask = BIT_MASK(nr);
unsigned long *p = ((unsigned long *)addr) + BIT_WORD(nr);
unsigned long old;
unsigned long flags;
_atomic_spin_lock_irqsave(p, flags);
old = *p;
*p = old ^ mask;
_atomic_spin_unlock_irqrestore(p, flags);
return (old & mask) != 0;
}
#endif /* _ASM_GENERIC_BITOPS_ATOMIC_H */
这就是所有的原子位操作,主要是通过位移动来实现的.