MINIX 防止中断重入的代码有问题，还是我错了？

[tanqing900920]

本人在学习MINIX，在做中断的时候，遇到一个百思不得其解的地方，minix 的代码
!*===========================================================================*
!* save *
!*===========================================================================*
! Save for protected mode.
! This is much simpler than for 8086 mode, because the stack already points
! into the process table, or has already been switched to the kernel stack.

.align 16
save:
cld ! set direction flag to a known value
pushad ! save "general" registers
o16 push ds ! save ds
o16 push es ! save es
o16 push fs ! save fs
o16 push gs ! save gs
mov dx, ss ! ss is kernel data segment
mov ds, dx ! load rest of kernel segments
mov es, dx ! kernel does not use fs, gs
mov eax, esp ! prepare to return
incb (_k_reenter) ! from -1 if not reentering
jnz set_restart1 ! stack is already kernel stack
mov esp, k_stktop
push _restart ! build return address for int handler
xor ebp, ebp ! for stacktrace
jmp RETADR-P_STACKBASE(eax)

.align 4
set_restart1:
push restart1
jmp RETADR-P_STACKBASE(eax)

;==============================我是分割线 ==================================；

;上面是保存CONTEXT的代码

;==============================我是分割线 ==================================；


! Note this is a macro, it looks like a subroutine.
#define hwint_master(irq) \
call save /* save interrupted process state */;\
inb INT_CTLMASK ;\
orb al, [1<<irq] ;\
outb INT_CTLMASK /* disable the irq */;\
movb al, ENABLE ;\
outb INT_CTL /* reenable master 8259 */;\
sti /* enable interrupts */;\
push irq /* irq */;\
call (_irq_table + 4*irq) /* eax = (*irq_table[irq])(irq) */;\
pop ecx ;\
cli /* disable interrupts */;\
test eax, eax /* need to reenable irq? */;\
jz 0f ;\
inb INT_CTLMASK ;\
andb al, ~[1<<irq] ;\
outb INT_CTLMASK /* enable the irq */;\
0: ret /* restart (another) process */

;==============================我又是分割线 ==================================；

;上面是8259A主片的宏
;==============================还是我 ==================================；

?????????????????????????

>>本人有一点很不明白百思不得其解，为什么只设置了一个(_k_reenter)，来防止中断重入，试想一下，要是我先是键盘中断产生了，键盘程序没处理完，这时候时钟中断发生，因为键盘中断没处理完，那么(_k_reenter) 没有 减1，时钟这时候CMP (_k_reenter)，0是不成立的，时钟ISR不能执行，这真的是滑天下之大稽了，难道他写的时候没发现这个有问题吗？还是我脑子进水了？