* Revert r42319 as it introduces a race condition when entering the kernel

debugger. As sInDebugger is already > 0 when the first CPU enters KDL, code
  from other CPUs might see debug_debugger_running() == true already before they
  enter the debugger.
* Instead, move the sDebuggerOnCPU setting out of the debugger loop and hold the
  value until after calling exit_kernel_debugger() so that the exit hooks still
  see debug_debugger_running() == true.
* Also avoid calling exit_kernel_debugger() when we've been called recursively
  (previousCPU != -1). Previously the exit hooks would've been called and the
  debugger state reset erroneously. To balance the missing decrement of
  sInDebugger in that case we decrement sInDebugger in enter_kernel_debugger()
  also when detecting the recursion case.


git-svn-id: file:///srv/svn/repos/haiku/haiku/trunk@42320 a95241bf-73f2-0310-859d-f6bbb57e9c96

src/system/kernel/debug/debug.cpp

@@ -805,9 +805,6 @@ static void
 kernel_debugger_loop(const char* messagePrefix, const char* message,
 	va_list args, int32 cpu)
 {
-	int32 previousCPU = sDebuggerOnCPU;
-	sDebuggerOnCPU = cpu;
-
 	DebugAllocPool* allocPool = create_debug_alloc_pool();
 
 	sCurrentKernelDebuggerMessagePrefix = messagePrefix;
@@ -936,8 +933,6 @@ kernel_debugger_loop(const char* messagePrefix, const char* message,
 		va_end(sCurrentKernelDebuggerMessageArgs);
 
 	delete_debug_alloc_pool(allocPool);
-
-	sDebuggerOnCPU = previousCPU;
 }
 
 
@@ -945,6 +940,8 @@ static void
 enter_kernel_debugger(int32 cpu)
 {
 	while (atomic_add(&sInDebugger, 1) > 0) {
+		atomic_add(&sInDebugger, -1);
+
 		// The debugger is already running, find out where...
 		if (sDebuggerOnCPU == cpu) {
 			// We are re-entering the debugger on the same CPU.
@@ -955,7 +952,6 @@ enter_kernel_debugger(int32 cpu)
 		// us. Process ICIs to ensure we get the halt request. Then we are
 		// blocking there until everyone leaves the debugger and we can
 		// try to enter it again.
-		atomic_add(&sInDebugger, -1);
 		smp_intercpu_int_handler(cpu);
 	}
 
@@ -1027,13 +1023,23 @@ kernel_debugger_internal(const char* messagePrefix, const char* message,
 		} else
 			enter_kernel_debugger(cpu);
 
+		// If we're called recursively sDebuggerOnCPU will be != -1.
+		int32 previousCPU = sDebuggerOnCPU;
+		sDebuggerOnCPU = cpu;
+
 		kernel_debugger_loop(messagePrefix, message, args, cpu);
 
-		if (sHandOverKDLToCPU < 0) {
+		if (sHandOverKDLToCPU < 0 && previousCPU == -1) {
+			// We're not handing over to a different CPU and we weren't
+			// called recursively, so we'll exit the debugger.
 			exit_kernel_debugger();
-			break;
 		}
 
+		sDebuggerOnCPU = previousCPU;
+
+		if (sHandOverKDLToCPU < 0)
+			break;
+
 		hand_over_kernel_debugger();
 
 		debug_trap_cpu_in_kdl(cpu, true);
@@ -1617,7 +1623,7 @@ debug_stop_screen_debug_output(void)
 bool
 debug_debugger_running(void)
 {
-	return sInDebugger > 0;
+	return sDebuggerOnCPU != -1;
 }