tracing: Don't trace kernel thread syscalls

Kernel threads don't call syscalls using the sysenter/sysexit
path. Instead they directly call the sys_* or do_* functions
that implement the syscalls inside the kernel.

The current syscall tracepoints only bind the sysenter/sysexit
path, then it has no effect to trace the kernel thread calls
to syscalls in that path.
Setting the TIF_SYSCALL_TRACEPOINT flag is then useless for these.

Actually there is only one case when a kernel thread can reach the
usual syscall exit tracing path: when we create a kernel thread, the
child comes to ret_from_fork and is the fork() return is then traced.
But this information alone is useless, then we don't want to set the
TIF flags for these threads.

Kernel threads have task_struct->mm set to NULL.
(Thanks to Heiko for that hint ;-)
The idea is then to check the mm field in syscall_regfunc() and
set the flag accordingly.

Signed-off-by: Hendrik Brueckner <[email protected]>
Cc: Jason Baron <[email protected]>
Cc: Frederic Weisbecker <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: Lai Jiangshan <[email protected]>
Cc: Steven Rostedt <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Mathieu Desnoyers <[email protected]>
Cc: Jiaying Zhang <[email protected]>
Cc: Martin Bligh <[email protected]>
Cc: Li Zefan <[email protected]>
Cc: Martin Schwidefsky <[email protected]>
Cc: Paul Mundt <[email protected]>
Cc: Heiko Carstens <[email protected]>
Cc: Hendrik Brueckner <[email protected]>
LKML-Reference: <[email protected]>
Signed-off-by: Frederic Weisbecker <[email protected]>

kernel/tracepoint.c

@@ -597,7 +597,9 @@ void syscall_regfunc(void)
 	if (!sys_tracepoint_refcount) {
 		read_lock_irqsave(&tasklist_lock, flags);
 		do_each_thread(g, t) {
-			set_tsk_thread_flag(t, TIF_SYSCALL_TRACEPOINT);
+			/* Skip kernel threads. */
+			if (t->mm)
+				set_tsk_thread_flag(t, TIF_SYSCALL_TRACEPOINT);
 		} while_each_thread(g, t);
 		read_unlock_irqrestore(&tasklist_lock, flags);
 	}