kgdb: Add the ability to schedule a breakpoint via a tasklet

Some kgdb I/O modules require the ability to create a breakpoint
tasklet, such as kgdboc and external modules such as kgdboe.  The
breakpoint tasklet is used as an asynchronous entry point into the
debugger which will have a different function scope than the current
execution path where it might not be safe to have an inline
breakpoint.  This is true of some of the kgdb I/O drivers which share
code with kgdb and rest of the kernel users.
Signed-off-by: Jason Wessel <jason.wessel@windriver.com>

include/linux/kgdb.h

@@ -271,6 +271,7 @@ extern int kgdb_mem2hex(char *mem, char *buf, int count);
 extern int kgdb_hex2mem(char *buf, char *mem, int count);
 
 extern int kgdb_isremovedbreak(unsigned long addr);
+extern void kgdb_schedule_breakpoint(void);
 
 extern int
 kgdb_handle_exception(int ex_vector, int signo, int err_code,

kernel/debug/debug_core.c

@@ -114,6 +114,7 @@ EXPORT_SYMBOL_GPL(kgdb_active);
  */
 static atomic_t			passive_cpu_wait[NR_CPUS];
 static atomic_t			cpu_in_kgdb[NR_CPUS];
+static atomic_t			kgdb_break_tasklet_var;
 atomic_t			kgdb_setting_breakpoint;
 
 struct task_struct		*kgdb_usethread;
@@ -789,6 +790,31 @@ static void kgdb_unregister_callbacks(void)
 	}
 }
 
+/*
+ * There are times a tasklet needs to be used vs a compiled in
+ * break point so as to cause an exception outside a kgdb I/O module,
+ * such as is the case with kgdboe, where calling a breakpoint in the
+ * I/O driver itself would be fatal.
+ */
+static void kgdb_tasklet_bpt(unsigned long ing)
+{
+	kgdb_breakpoint();
+	atomic_set(&kgdb_break_tasklet_var, 0);
+}
+
+static DECLARE_TASKLET(kgdb_tasklet_breakpoint, kgdb_tasklet_bpt, 0);
+
+void kgdb_schedule_breakpoint(void)
+{
+	if (atomic_read(&kgdb_break_tasklet_var) ||
+		atomic_read(&kgdb_active) != -1 ||
+		atomic_read(&kgdb_setting_breakpoint))
+		return;
+	atomic_inc(&kgdb_break_tasklet_var);
+	tasklet_schedule(&kgdb_tasklet_breakpoint);
+}
+EXPORT_SYMBOL_GPL(kgdb_schedule_breakpoint);
+
 static void kgdb_initial_breakpoint(void)
 {
 	kgdb_break_asap = 0;