[PATCH 17/23] tracehook: force signal_pending()

[Roland McGrath <roland@redhat.com>]

This defines a new hook tracehook_force_sigpending() that lets
tracing code decide to force TIF_SIGPENDING on in recalc_sigpending().

This is not used yet, so it compiles away to nothing for now.
It lays the groundwork for new tracing code that can interrupt
a task synthetically without actually sending a signal.

Signed-off-by: Roland McGrath <roland@redhat.com>
---
 include/linux/tracehook.h |   14 ++++++++++++++
 kernel/signal.c           |    4 +++-
 2 files changed, 17 insertions(+), 1 deletions(-)

diff --git a/include/linux/tracehook.h b/include/linux/tracehook.h
index 4c50e1b..43bc51b 100644
--- a/include/linux/tracehook.h
+++ b/include/linux/tracehook.h
@@ -423,6 +423,20 @@ static inline int tracehook_consider_fatal_signal(struct task_struct *task,
 }
 
 /**
+ * tracehook_force_sigpending - let tracing force signal_pending(current) on
+ *
+ * Called when recomputing our signal_pending() flag.  Return nonzero
+ * to force the signal_pending() flag on, so that tracehook_get_signal()
+ * will be called before the next return to user mode.
+ *
+ * Called with @current->sighand->siglock held.
+ */
+static inline int tracehook_force_sigpending(void)
+{
+	return 0;
+}
+
+/**
  * tracehook_get_signal - deliver synthetic signal to traced task
  * @task:		@current
  * @regs:		task_pt_regs(@current)
diff --git a/kernel/signal.c b/kernel/signal.c
index cbec18e..44b5823 100644
--- a/kernel/signal.c
+++ b/kernel/signal.c
@@ -134,7 +134,9 @@ void recalc_sigpending_and_wake(struct task_struct *t)
 
 void recalc_sigpending(void)
 {
-	if (!recalc_sigpending_tsk(current) && !freezing(current))
+	if (unlikely(tracehook_force_sigpending()))
+		set_thread_flag(TIF_SIGPENDING);
+	else if (!recalc_sigpending_tsk(current) && !freezing(current))
 		clear_thread_flag(TIF_SIGPENDING);
 
 }