[PATCH] x86_64: Keep only a single debug notifier chain

Calling a notifier three times in the debug handler does not make much sense,
because a debugger can figure out the various conditions by itself.  Remove
the additional calls to DIE_DEBUG and DIE_DEBUGSTEP completely.

This matches what i386 does now.

This also makes sure interrupts are always still disabled when calling a
debugger, which prevents:

BUG: using smp_processor_id() in preemptible [00000001] code: tpopf/1470
caller is post_kprobe_handler+0x9/0x70

Call Trace:<ffffffff8024f10f>{smp_processor_id+191} <ffffffff80120e69>{post_kpro
be_handler+9} 
<ffffffff80120f7a>{kprobe_exceptions_notify+58} 
<ffffffff80144fc0>{notifier_call_chain+32} <ffffffff80110daf>{do_debug+335} 
<ffffffff8010f513>{debug+127}  <EOE> 

on preemptible debug kernels with kprobes when single stepping in user space.

This was probably a bug even on non preempt kernels, this function was
supposed to be running with interrupts off according to a comment there.

Note to third part debugger maintainers: please double check your debugger can
still single step.

Cc: <prasanna@in.ibm.com>
Cc: <jbeulich@novell.com>
Cc: <kaos@sgi.com>
Cc: <jim.houston@ccur.com>
Cc: <jfv@bluesong.net>
Signed-off-by: Andi Kleen <ak@suse.de>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/x86_64/kernel/traps.c

@@ -659,9 +659,9 @@ asmlinkage void do_debug(struct pt_regs * regs, unsigned long error_code)
 	asm("movq %%db6,%0" : "=r" (condition));
 
 	if (notify_die(DIE_DEBUG, "debug", regs, condition, error_code,
-						SIGTRAP) == NOTIFY_STOP) {
+						SIGTRAP) == NOTIFY_STOP)
 		return;
-	}
+
 	conditional_sti(regs);
 
 	/* Mask out spurious debug traps due to lazy DR7 setting */
@@ -674,9 +674,7 @@ asmlinkage void do_debug(struct pt_regs * regs, unsigned long error_code)
 	tsk->thread.debugreg6 = condition;
 
 	/* Mask out spurious TF errors due to lazy TF clearing */
-	if ((condition & DR_STEP) &&
+	if (condition & DR_STEP) {
-	    (notify_die(DIE_DEBUGSTEP, "debugstep", regs, condition,
-			1, SIGTRAP) != NOTIFY_STOP)) {
 		/*
 		 * The TF error should be masked out only if the current
 		 * process is not traced and if the TRAP flag has been set
@@ -711,16 +709,10 @@ asmlinkage void do_debug(struct pt_regs * regs, unsigned long error_code)
 	force_sig_info(SIGTRAP, &info, tsk);	
 clear_dr7:
 	asm volatile("movq %0,%%db7"::"r"(0UL));
-	notify_die(DIE_DEBUG, "debug", regs, condition, 1, SIGTRAP);
 	return;
 
 clear_TF_reenable:
 	set_tsk_thread_flag(tsk, TIF_SINGLESTEP);
-
-clear_TF:
-	/* RED-PEN could cause spurious errors */
-	if (notify_die(DIE_DEBUG, "debug2", regs, condition, 1, SIGTRAP) 
-								!= NOTIFY_STOP)
 	regs->eflags &= ~TF_MASK;
 }
 

include/asm-x86_64/kdebug.h

@@ -23,7 +23,6 @@ enum die_val {
 	DIE_OOPS = 1,
 	DIE_INT3,
 	DIE_DEBUG,
-	DIE_DEBUGSTEP,
 	DIE_PANIC,
 	DIE_NMI,
 	DIE_DIE,