/** low level task data that entry.S needs immediate access to.* __switch_to() assumes cpu_context follows immediately after cpu_domain.*/
struct thread_info {unsigned long       flags;      /* low level flags */mm_segment_t       addr_limit; /* address limit */struct task_struct   *task;      /* main task structure */struct exec_domain *exec_domain;   /* execution domain */struct restart_block  restart_block;int           preempt_count;  /* 0 => preemptable, <0 => bug */int         cpu;        /* cpu */
};

在支持可抢占的系统中，一个进程的therad_info信息定义如上。其中preempt_count代表的是该进程是否可以被抢占，根据注释的说明当peermpt_count等于0的时候当前进程就可以被抢占，当小于0存在bug，当不等于0也就是大于0说明当前进程不可以被抢占。不可抢占的原因很多，比如当前进程在中断上下文中或者使用了锁（spin_lock的过程中会disable掉抢占的）。至于当前是什么原因不能被抢占，就需要看peermpt_count每个字段的含义。

字段含义

<linux/include/preempt_mask.h>
------------------------------------------
/** We put the hardirq and softirq counter into the preemption* counter. The bitmask has the following meaning:** - bits 0-7 are the preemption count (max preemption depth: 256)* - bits 8-15 are the softirq count (max # of softirqs: 256)** The hardirq count could in theory be the same as the number of* interrupts in the system, but we run all interrupt handlers with* interrupts disabled, so we cannot have nesting interrupts. Though* there are a few palaeontologic drivers which reenable interrupts in* the handler, so we need more than one bit here.** PREEMPT_MASK:   0x000000ff* SOFTIRQ_MASK:   0x0000ff00* HARDIRQ_MASK:   0x000f0000*     NMI_MASK:   0x00100000* PREEMPT_ACTIVE: 0x00200000*/
#define PREEMPT_BITS    8
#define SOFTIRQ_BITS    8
#define HARDIRQ_BITS    4
#define NMI_BITS    1

结合上述的示图和代码的定义可知，bit0-7代表的是抢占的次数，最大抢占深度为256次； bit8-15代表的是软中断的次数，最大也是256次； bit16-19表示硬件中断的次数，注释的大概意思是避免中断嵌套，但是也不能防止某些驱动中嵌套使用中断，所以嵌套16层也是最大次数了。bit20代表NMI中断，bit21代表当前抢占是否active。

#define PREEMPT_SHIFT   0
#define SOFTIRQ_SHIFT   (PREEMPT_SHIFT + PREEMPT_BITS)                        //0+8=8
#define HARDIRQ_SHIFT   (SOFTIRQ_SHIFT + SOFTIRQ_BITS)                        //8+8=16
#define NMI_SHIFT   (HARDIRQ_SHIFT + HARDIRQ_BITS)                        //16+4=20#define __IRQ_MASK(x) ((1UL << (x))-1)#define PREEMPT_MASK  (__IRQ_MASK(PREEMPT_BITS) << PREEMPT_SHIFT)
#define SOFTIRQ_MASK    (__IRQ_MASK(SOFTIRQ_BITS) << SOFTIRQ_SHIFT)
#define HARDIRQ_MASK    (__IRQ_MASK(HARDIRQ_BITS) << HARDIRQ_SHIFT)
#define NMI_MASK    (__IRQ_MASK(NMI_BITS)     << NMI_SHIFT)#define PREEMPT_OFFSET (1UL << PREEMPT_SHIFT)                //1<<0
#define SOFTIRQ_OFFSET  (1UL << SOFTIRQ_SHIFT)                //1<<8
#define HARDIRQ_OFFSET  (1UL << HARDIRQ_SHIFT)                //1<<16
#define NMI_OFFSET  (1UL << NMI_SHIFT)                    //1<<20#define SOFTIRQ_DISABLE_OFFSET (2 * SOFTIRQ_OFFSET)           //16#define PREEMPT_ACTIVE_BITS  1
#define PREEMPT_ACTIVE_SHIFT    (NMI_SHIFT + NMI_BITS)
#define PREEMPT_ACTIVE  (__IRQ_MASK(PREEMPT_ACTIVE_BITS) << PREEMPT_ACTIVE_SHIFT)#define hardirq_count()  (preempt_count() & HARDIRQ_MASK)                                     //硬中断count
#define softirq_count() (preempt_count() & SOFTIRQ_MASK)                                     //软中断count
#define irq_count() (preempt_count() & (HARDIRQ_MASK | SOFTIRQ_MASK \| NMI_MASK))                                                  //所有中断=硬+软+NMI

从上述的定义可以得出，如果想知道硬中断的次数就使用hardirq_count， 如果想知道软中断次数就使用softirq_count， 如果想知道所有中断的次数就使用irq_count。

/** 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)

其中in_irq用于判断当前进程是否在硬中断中；  in_softirq用于判断是否当前进程在软件中断或者有别的进程disable了软中断

#define in_atomic()  ((preempt_count() & ~PREEMPT_ACTIVE) != 0)

使用上述的宏判断当前进程是否处于原子操作中。