[PATCH] show_task() is not SMP safe

From: Arnd Bergmann <arnd@arndb.de>

Christian Bornträger noticed that the kernel can crash after <SysRq>-T.  It
appears that the show_task function gets called for all tasks, which does
not work if one of the tasks is running in a system call on another CPU.
In that case the result of thread_saved_pc and show_stack is undefined and
likely to cause a crash.

For tasks running in user space on other CPUs, show_task() is probably
harmless, but I'm not sure if that's true on all architectures.

The patch below is still racy for tasks that are about to sleep, but it
demonstrates the problem.

kernel/sched.c

@@ -2580,13 +2580,13 @@ static void show_task(task_t * p)
 	else
 		printk("?");
 #if (BITS_PER_LONG == 32)
-	if (p == current)
-		printk(" current  ");
+	if (state == TASK_RUNNING)
+		printk(" running ");
 	else
 		printk(" %08lX ", thread_saved_pc(p));
 #else
-	if (p == current)
-		printk("   current task   ");
+	if (state == TASK_RUNNING)
+		printk("  running task   ");
 	else
 		printk(" %016lx ", thread_saved_pc(p));
 #endif
@@ -2608,7 +2608,8 @@ static void show_task(task_t * p)
 	else
 		printk(" (NOTLB)\n");
 
-	show_stack(p, NULL);
+	if (state != TASK_RUNNING)
+		show_stack(p, NULL);
 }
 
 void show_state(void)