[PATCH] CONFIG_PREEMPT fix of do_debug()

If CONFIG_PREEMPT is enabled, and the kernel is preempted just before
do_debug() has a chance to save the debug register values, DR6 could be
read from the wrong CPU.

It is exactly the same problem as reading %cr2 in the page fault
handler.  Same fix: make the handler a interrupt gate, and enable
interrupts only once safe.

arch/i386/kernel/traps.c

@@ -514,6 +514,10 @@ asmlinkage void do_debug(struct pt_regs * regs, long error_code)
 
 	__asm__ __volatile__("movl %%db6,%0" : "=r" (condition));
 
+	/* It's safe to allow irq's after DR6 has been saved */
+	if (regs->eflags & X86_EFLAGS_IF)
+		local_irq_enable();
+
 	/* Mask out spurious debug traps due to lazy DR7 setting */
 	if (condition & (DR_TRAP0|DR_TRAP1|DR_TRAP2|DR_TRAP3)) {
 		if (!tsk->thread.debugreg[7])
@@ -831,7 +835,7 @@ void __init trap_init(void)
 #endif
 
 	set_trap_gate(0,&divide_error);
-	set_trap_gate(1,&debug);
+	set_intr_gate(1,&debug);
 	set_intr_gate(2,&nmi);
 	set_system_gate(3,&int3);	/* int3-5 can be called from all */
 	set_system_gate(4,&overflow);