[PATCH v2 3/3] tracing: syscall_regfunc() should not skip kernel threads

[Oleg Nesterov <oleg@redhat.com>]

syscall_regfunc() ignores the kernel threads because "it has no effect",
see cc3b13c1 "Don't trace kernel thread syscalls" which added this check.

However, this means that a user-space task spawned by call_usermodehelper()
will run without TIF_SYSCALL_TRACEPOINT if sys_tracepoint_refcount != 0.

Remove this check. The unnecessary report from ret_from_fork path mentioned
by cc3b13c1 is no longer possible, see See commit fb45550d76bb5 "make sure
that kernel_thread() callbacks call do_exit() themselves".

A kernel_thread() callback can only return and take the int_ret_from_sys_call
path after do_execve() succeeds, otherwise the kernel will crash. But in this
case it is no longer a kernel thread and thus is needs TIF_SYSCALL_TRACEPOINT.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
---
 kernel/tracepoint.c |    4 +---
 1 files changed, 1 insertions(+), 3 deletions(-)

diff --git a/kernel/tracepoint.c b/kernel/tracepoint.c
index d907b7b..08456ba 100644
--- a/kernel/tracepoint.c
+++ b/kernel/tracepoint.c
@@ -747,9 +747,7 @@ void syscall_regfunc(void)
 	if (!sys_tracepoint_refcount) {
 		read_lock(&tasklist_lock);
 		for_each_process_thread(p, t) {
-			/* Skip kernel threads. */
-			if (!(t->flags & PF_KTHREAD))
-				set_tsk_thread_flag(t, TIF_SYSCALL_TRACEPOINT);
+			set_tsk_thread_flag(t, TIF_SYSCALL_TRACEPOINT);
 		}
 		read_unlock(&tasklist_lock);
 	}
-- 
1.5.5.1