eventfd: Add eventfd_signal()

The `eventfd_signal()` function is the equivalent to a write to an
eventfd file descriptor: it bumps the internal counter and wakes up
processes waiting for it.

`eventfd_signal()` is meant to be used by kernel drivers. DRM drivers
will call it through linuxkpi.

Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50850

sys/kern/sys_eventfd.c

@@ -157,6 +157,27 @@ eventfd_put(struct eventfd *efd)
 	free(efd, M_EVENTFD);
 }
 
+static void
+eventfd_wakeup(struct eventfd *efd)
+{
+	KNOTE_LOCKED(&efd->efd_sel.si_note, 0);
+	selwakeup(&efd->efd_sel);
+	wakeup(&efd->efd_count);
+}
+
+void
+eventfd_signal(struct eventfd *efd)
+{
+	mtx_lock(&efd->efd_lock);
+
+	if (efd->efd_count < UINT64_MAX)
+		efd->efd_count++;
+
+	eventfd_wakeup(efd);
+
+	mtx_unlock(&efd->efd_lock);
+}
+
 static int
 eventfd_close(struct file *fp, struct thread *td)
 {
@@ -244,9 +265,7 @@ eventfd_write(struct file *fp, struct uio *uio, struct ucred *active_cred,
 	if (error == 0) {
 		MPASS(UINT64_MAX - efd->efd_count > count);
 		efd->efd_count += count;
-		KNOTE_LOCKED(&efd->efd_sel.si_note, 0);
-		selwakeup(&efd->efd_sel);
-		wakeup(&efd->efd_count);
+		eventfd_wakeup(efd);
 	}
 	mtx_unlock(&efd->efd_lock);
 

sys/sys/eventfd.h

@@ -44,6 +44,7 @@ int eventfd_create_file(struct thread *td, struct file *fp, uint32_t initval,
     int flags);
 struct eventfd *eventfd_get(struct file *fp);
 void eventfd_put(struct eventfd *efd);
+void eventfd_signal(struct eventfd *efd);
 
 #else