preempt_count详解

当从内核态返回到用户态的时候，要检查是否进行调度，而调度要看两个条件：

1.preempt_count是否为0

2.rescheduled是否置位

 

ret_from_exception:
 preempt_stop(CLBR_ANY)
ret_from_intr:
 GET_THREAD_INFO(%ebp)
check_userspace:
 movl PT_EFLAGS(%esp), %eax # mix EFLAGS and CS
 movb PT_CS(%esp), %al
 andl $(VM_MASK | SEGMENT_RPL_MASK), %eax
 cmpl $USER_RPL, %eax
 jb resume_kernel  # not returning to v8086 or userspace

ENTRY(resume_userspace)
 LOCKDEP_SYS_EXIT
  DISABLE_INTERRUPTS(CLBR_ANY) # make sure we don't miss an interrupt
     # setting need_resched or sigpending
     # between sampling and the iret
 movl TI_flags(%ebp), %ecx
 andl $_TIF_WORK_MASK, %ecx # is there any work to be done on
     # int/exception return?
 jne work_pending
 jmp restore_all
END(ret_from_exception)

#ifdef CONFIG_PREEMPT
ENTRY(resume_kernel)
 DISABLE_INTERRUPTS(CLBR_ANY)
 cmpl $0,TI_preempt_count(%ebp) # non-zero preempt_count ?
 jnz restore_nocheck
need_resched:
 movl TI_flags(%ebp), %ecx # need_resched set ?
 testb $_TIF_NEED_RESCHED, %cl
 jz restore_all
 testl $IF_MASK,PT_EFLAGS(%esp) # interrupts off (exception path) ?
 jz restore_all
 call preempt_schedule_irq
 jmp need_resched
END(resume_kernel)
#endif
 CFI_ENDPROC

检查preempt_count的时候，是统一检查是否为0，也就是说，有4个条件限制，可能不能够进行调度。

1.preempt_disable()

 

#define preempt_disable() \
do { \
 inc_preempt_count(); \
 barrier(); \
} while (0)

 

#define inc_preempt_count() add_preempt_count(1)

 

会在preempt_enable中释放

 

2.add_preempt_count(HARDIRQ_OFFSET)

是在irq_enter()中调用的，记录进入硬件中断处理的次数（计数），

会在irq_exit()中释放

 

3.__local_bh_disable((unsigned long)__builtin_return_address(0));

在do_softirq中调用的，

static inline void __local_bh_disable(unsigned long ip)
{
 add_preempt_count(SOFTIRQ_OFFSET);
 barrier();
}

 

4.总开关，第一位。