抢占计数的简单注释

在x86里面irq的入口函数是 do_irq()

在arm里面irq的入口函数是 asm_do_irq()

 CONFIG_IRQ_FORCED_THREADING  ? 什么用途?

------------------------------
#define PREEMPT_BITS 8
#define SOFTIRQ_BITS 8
#define NMI_BITS 1

#define MAX_HARDIRQ_BITS 10

#define PREEMPT_SHIFT 0
#define SOFTIRQ_SHIFT 8
#define HARDIRQ_SHIFT 16
#define NMI_SHIFT        26

#define __IRQ_MASK(x) ((1UL << (x))-1)

#define PREEMPT_MASK (__IRQ_MASK(PREEMPT_BITS) << PREEMPT_SHIFT)         -------->     0x000000ff
#define SOFTIRQ_MASK (__IRQ_MASK(SOFTIRQ_BITS) << SOFTIRQ_SHIFT)        -------->     0x0000ff00
#define HARDIRQ_MASK (__IRQ_MASK(HARDIRQ_BITS) << HARDIRQ_SHIFT)        -------->     0x03ff0000
#define NMI_MASK (__IRQ_MASK(NMI_BITS)     << NMI_SHIFT)   ------>0x04000000

#define PREEMPT_OFFSET (1UL <<0)
#define SOFTIRQ_OFFSET (1UL << 8)
#define HARDIRQ_OFFSET (1UL << 16)
#define NMI_OFFSET  (1UL << 26)

#define hardirq_count() (preempt_count() & HARDIRQ_MASK)
#define softirq_count() (preempt_count() & SOFTIRQ_MASK)
#define irq_count() (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_MASK \
     | NMI_MASK))  /* 软中断/硬中断/NMI上任何bit有指示， 都会体现到in_interrupt（）  */

/*
 * Are we doing bottom half or hardware interrupt processing?
 * Are we in a softirq context? Interrupt context?
 * in_softirq - Are we currently processing softirq or have bh disabled?
 * in_serving_softirq - Are we currently processing softirq?
 */
#define in_irq()  (hardirq_count())
#define in_softirq()  (softirq_count())
#define in_interrupt()  (irq_count())     /*  */
#define in_serving_softirq() (softirq_count() & SOFTIRQ_OFFSET)

/*
 * Are we in NMI context?
 */
#define in_nmi() (preempt_count() & NMI_MASK)

--NMI-------------hardirq-----------------------softirq--------------preempt--------
-----27--26--------------------------16--15------------------8--7---------------------0
        |                              |                      |                       |
----------------------------------------------------------------------------------------

#define SOFTIRQ_DISABLE_OFFSET (2 * SOFTIRQ_OFFSET) 
               =(1UL << 9)=512 使用bit 9作为softirq是否关闭的标志，因为softirq是不可嵌套的，所以softirq的计数使用一个bit， 即bit8就可以了。 bit8指示计数， bit9指示softirq是否关闭
softirq使用个两个bit是不是就足够了？

hardirq是否也是不可嵌套的？如果不可嵌套，那么是否一个bit就够了。 关闭硬中断， 不依赖于hardirq_count，通过设置 硬件寄存器实现硬中断的关闭和打开。