signal: sigprocmask: narrow the scope of ->siglock

No functional changes, preparation to simplify the review of the next change.

1. We can read current->block lockless, nobody else can ever change this mask.

2. Calculate the resulting sigset_t outside of ->siglock into the temporary
   variable, then take ->siglock and change ->blocked.

Also, kill the stale comment about BKL.

Signed-off-by: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Matt Fleming <matt.fleming@linux.intel.com>
Acked-by: Tejun Heo <tj@kernel.org>

kernel/signal.c

@@ -2299,12 +2299,6 @@ long do_no_restart_syscall(struct restart_block *param)
 	return -EINTR;
 }
 
-/*
- * We don't need to get the kernel lock - this is all local to this
- * particular thread.. (and that's good, because this is _heavily_
- * used by various programs)
- */
-
 /*
  * This is also useful for kernel threads that want to temporarily
  * (or permanently) block certain signals.
@@ -2315,30 +2309,33 @@ long do_no_restart_syscall(struct restart_block *param)
  */
 int sigprocmask(int how, sigset_t *set, sigset_t *oldset)
 {
-	int error;
+	struct task_struct *tsk = current;
+	sigset_t newset;
 
-	spin_lock_irq(&current->sighand->siglock);
+	/* Lockless, only current can change ->blocked, never from irq */
 	if (oldset)
-		*oldset = current->blocked;
+		*oldset = tsk->blocked;
 
-	error = 0;
 	switch (how) {
 	case SIG_BLOCK:
-		sigorsets(&current->blocked, &current->blocked, set);
+		sigorsets(&newset, &tsk->blocked, set);
 		break;
 	case SIG_UNBLOCK:
-		signandsets(&current->blocked, &current->blocked, set);
+		signandsets(&newset, &tsk->blocked, set);
 		break;
 	case SIG_SETMASK:
-		current->blocked = *set;
+		newset = *set;
 		break;
 	default:
-		error = -EINVAL;
+		return -EINVAL;
 	}
+
+	spin_lock_irq(&tsk->sighand->siglock);
+	tsk->blocked = newset;
 	recalc_sigpending();
-	spin_unlock_irq(&current->sighand->siglock);
+	spin_unlock_irq(&tsk->sighand->siglock);
 
-	return error;
+	return 0;
 }
 
 /**