eventfd: Add refcounting

An eventfd file descriptor can be used by drivers such as DRM drivers
through linuxkpi. A driver can hold a reference to such a file
regardless of the fact it is used by userland or not.

This patch introduces a refcount in `struct eventfd`, plus the
`eventfd_get()` and `eventfd_put()` functions to acquire and release
references. These functions will be used by DRM drivers for instance.

This structure is internal to `sys/kern/sys_eventfd.c` and not used
anywhere else. Thus it is safe to add a field without breaking anything.

Reviewed by:	markj
Sponsored by:	The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50849

sys/kern/sys_eventfd.c

@@ -40,6 +40,7 @@
 #include <sys/mutex.h>
 #include <sys/poll.h>
 #include <sys/proc.h>
+#include <sys/refcount.h>
 #include <sys/selinfo.h>
 #include <sys/stat.h>
 #include <sys/uio.h>
@@ -102,6 +103,7 @@ struct eventfd {
 	uint32_t	efd_flags;
 	struct selinfo	efd_sel;
 	struct mtx	efd_lock;
+	unsigned int	efd_refcount;
 };
 
 int
@@ -119,6 +121,7 @@ eventfd_create_file(struct thread *td, struct file *fp, uint32_t initval,
 	efd->efd_count = initval;
 	mtx_init(&efd->efd_lock, "eventfd", NULL, MTX_DEF);
 	knlist_init_mtx(&efd->efd_sel.si_note, &efd->efd_lock);
+	refcount_init(&efd->efd_refcount, 1);
 
 	fflags = FREAD | FWRITE;
 	if ((flags & EFD_NONBLOCK) != 0)
@@ -128,16 +131,39 @@ eventfd_create_file(struct thread *td, struct file *fp, uint32_t initval,
 	return (0);
 }
 
+struct eventfd *
+eventfd_get(struct file *fp)
+{
+	struct eventfd *efd;
+
+	if (fp->f_data == NULL || fp->f_ops != &eventfdops)
+		return (NULL);
+
+	efd = fp->f_data;
+	refcount_acquire(&efd->efd_refcount);
+
+	return (efd);
+}
+
+void
+eventfd_put(struct eventfd *efd)
+{
+	if (!refcount_release(&efd->efd_refcount))
+		return;
+
+	seldrain(&efd->efd_sel);
+	knlist_destroy(&efd->efd_sel.si_note);
+	mtx_destroy(&efd->efd_lock);
+	free(efd, M_EVENTFD);
+}
+
 static int
 eventfd_close(struct file *fp, struct thread *td)
 {
 	struct eventfd *efd;
 
 	efd = fp->f_data;
-	seldrain(&efd->efd_sel);
-	knlist_destroy(&efd->efd_sel.si_note);
-	mtx_destroy(&efd->efd_lock);
-	free(efd, M_EVENTFD);
+	eventfd_put(efd);
 	return (0);
 }
 

sys/sys/eventfd.h

@@ -38,8 +38,12 @@ typedef uint64_t eventfd_t;
 
 #ifdef _KERNEL
 
+struct eventfd;
+
 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);
 
 #else