kernel/signal.c: simplify force_sig_info_to_task(), kill recalc_sigpending_and_wake()

The purpose of recalc_sigpending_and_wake() is not clear, it looks
"obviously unneeded" because we are going to send the signal which can't
be blocked or ignored.

Add the comment to explain why we can't rely on send_signal_locked() and
make this logic more simple/explicit.  recalc_sigpending_and_wake() has no
other users, it can die.

In fact I think we don't even need signal_wake_up(), the target task must
be either current or a TASK_TRACED child, otherwise the usage of siglock
is not safe.  But this needs another change.

Link: https://lkml.kernel.org/r/20231120151649.GA15995@redhat.comSigned-off-by: Oleg Nesterov <oleg@redhat.com>
Cc: Eric Biederman <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/sched/signal.h

@@ -432,7 +432,6 @@ static inline bool fault_signal_pending(vm_fault_t fault_flags,
  * This is required every time the blocked sigset_t changes.
  * callers must hold sighand->siglock.
  */
-extern void recalc_sigpending_and_wake(struct task_struct *t);
 extern void recalc_sigpending(void);
 extern void calculate_sigpending(void);
 

kernel/signal.c

@@ -171,16 +171,6 @@ static bool recalc_sigpending_tsk(struct task_struct *t)
 	return false;
 }
 
-/*
- * After recalculating TIF_SIGPENDING, we need to make sure the task wakes up.
- * This is superfluous when called on current, the wakeup is a harmless no-op.
- */
-void recalc_sigpending_and_wake(struct task_struct *t)
-{
-	if (recalc_sigpending_tsk(t))
-		signal_wake_up(t, 0);
-}
-
 void recalc_sigpending(void)
 {
 	if (!recalc_sigpending_tsk(current) && !freezing(current))
@@ -1348,10 +1338,8 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t,
 		action->sa.sa_handler = SIG_DFL;
 		if (handler == HANDLER_EXIT)
 			action->sa.sa_flags |= SA_IMMUTABLE;
-		if (blocked) {
+		if (blocked)
 			sigdelset(&t->blocked, sig);
-			recalc_sigpending_and_wake(t);
-		}
 	}
 	/*
 	 * Don't clear SIGNAL_UNKILLABLE for traced tasks, users won't expect
@@ -1361,6 +1349,9 @@ force_sig_info_to_task(struct kernel_siginfo *info, struct task_struct *t,
 	    (!t->ptrace || (handler == HANDLER_EXIT)))
 		t->signal->flags &= ~SIGNAL_UNKILLABLE;
 	ret = send_signal_locked(sig, info, t, PIDTYPE_PID);
+	/* This can happen if the signal was already pending and blocked */
+	if (!task_sigpending(t))
+		signal_wake_up(t, 0);
 	spin_unlock_irqrestore(&t->sighand->siglock, flags);
 
 	return ret;