Manuel Bouyer <bouyer@antioche.lip6.fr> writes:

> Let's look at softclock() (from kern/clock.c):
>         while ((c = calltodo.c_next) != NULL && c->c_time <= 0) {
> 		func = c->c_func;
> 		arg = c->c_arg;
> 		calltodo.c_next = c->c_next;
> 		c->c_next = callfree;
> 		callfree = c;
> 		splx(s);
> 		(*func)(arg);
> 		(void) splhigh();
> 	}
> 
> My theory is based on the fact that c_next is not declared volatile, so
> calltodo.c_next may be cached in a register and not re-read from memory
> in the next iteration.

Since there is a function call between each test of calltodo.c_next,
the sane compiler shouldn't assume it can be cached.  At least 1.4.2
compiler wit -O2 generates following assember code and it looks like
doing memory access each time.

   583	.globl _softclock
   584		.type	 _softclock,@function
   585	_softclock:
   586		pushl %ebp
	:
   597		movl _calltodo,%edx
   598		movl %ecx,%edi
   599		notl %edi
   600		testl %edx,%edx
   601		je L178
   602		.align 2,0x90
   603	L184:
   604		cmpl $0,12(%edx)
   605		jg L178
	:
   618		call _Xspllower
   619	L182:
   620		pushl %ebx
   621		call *%esi
   622		addl $4,%esp
   623		movl _cpl,%eax
   624		orl _imask+4,%eax
   625		movl %eax,_cpl
   626		movl _calltodo,%edx
   627		testl %edx,%edx
   628		jne L184
   629	L178:
	:

enami.