show_task() is not SMP safe

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.

In the same function, there is another (harmless) bug that causes the "free stack" indicator to be wrong. It can take any value between zero and the intended meaning unless __alloc_thread_info is modified to clear newly allocated stack memory.

	Arnd <><

Index: kernel/sched.c
===================================================================
RCS file: /home/cvs/linux-2.5/kernel/sched.c,v
retrieving revision 1.56

diff -u -r1.56 sched.c
@@ -2457,13 +2457,13 @@
 	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

@@ -2491,7 +2491,8 @@
 	else
 		printk(" (NOTLB)\n");
 
-	show_stack(p, NULL);
+	if (state != TASK_RUNNING)
+		show_stack(p, NULL);
 }
 
 void show_state(void)