debug registers not working? 2.0 kernel

I'm experimenting with the debug registers in the 2.0 kernel. My goal is to implement data watchpoints. This work seems to have been done elsewhere and I plan to use that work, but in the meantime I'm just experimenting. I want to see a data watchpoint work. so far it hasn't. Can anyone help me figure out what I'm doing wrong?

I have verified that asm("int 1") results in the "debug" isr running. If I understand correctly, setting the debug registers in a certain way should result in interrupt line 1 firing. so far I haven't seen it work that way. is there something I'm not getting?

Thanks a lot!

- Ben

details:
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I added the following code to the top of the schedule() function as a simple test. I removed code elsewhere in the kernel that messes with (disables) %db7, and part of the following verifies that %db7 is never changed after I set it. I think software interrupt 1 should fire when 'has_run_2' is sampled, and probably again when it's updated, but it doesn't work. I've tried many many variations of this.

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static int odb7 = 0;
static int oaddr0 = 0;

int db7 = 0;
int addr0 = 0;

static int has_run = 0;
static unsigned long has_run_2 = 0;

asm ( " movl %%db7, %0\n"

      "        movl %%db0, %1\n"
     :"=r"(db7), "r="(addr0) );

if( db7 != odb7 || addr0 != oaddr0 )

{
	printk(KERN_DEBUG
		"%s: change: a0:0x%8.8x oa0:0x%8.8x "
		"db7:0x%8.8x odb7:0x%8.8x\n",
		__FUNCTION__, addr0, oaddr0, db7, odb7);

	odb7 = db7;
	oaddr0 = addr0;
}

if( ! has_run && jiffies > 7000 )

{
	has_run = 1;

	asm ("  movl %0, %%db0\n"
	     "  movl %1, %%db7\n"
	     : /* no inputs */
	     :"r"(&has_run_2),
	      "r"(0x000f0202)  /*LEN0=3 R/W0=3 GE=1 G0=1 */
	    );
}

if( has_run && jiffies > 8000 )

{
	/* read has_run_2 should generate 'int 1' */
	if( ! has_run_2 )
	{
		printk(KERN_DEBUG
		"%s: tested and now setting has_run_2\n",
		__FUNCTION__);

                /* write has_run_2 should generate 'int 1' */
		has_run_2 = 1;
	}
}