From 11ee9f36b21313f833babb0a72c92de3319da057 Mon Sep 17 00:00:00 2001
From: Taylor R Campbell <riastradh@NetBSD.org>
Date: Thu, 24 Feb 2022 15:53:11 +0000
Subject: [PATCH 49/49] WIP: kern_descrip.c: Membar audit.

---
 sys/kern/kern_descrip.c | 21 ++++++++++++++++++---
 1 file changed, 18 insertions(+), 3 deletions(-)

diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
index 1336ea9b8010..a1fa3bcdbdeb 100644
--- a/sys/kern/kern_descrip.c
+++ b/sys/kern/kern_descrip.c
@@ -392,6 +392,9 @@ fd_getfile(unsigned fd)
 		 * Multi threaded: issue a memory barrier to ensure that we
 		 * acquire the file pointer _after_ adding a reference.  If
 		 * no memory barrier, we could fetch a stale pointer.
+		 *
+		 * XXX Unclear what stale pointer this refers to...  I
+		 * don't think this membar is actually necessary.
 		 */
 		atomic_inc_uint(&ff->ff_refcnt);
 #ifndef __HAVE_ATOMIC_AS_MEMBAR
@@ -451,7 +454,7 @@ fd_putfile(unsigned fd)
 	 * CPU.
 	 */
 #ifndef __HAVE_ATOMIC_AS_MEMBAR
-	membar_exit();
+	membar_release();
 #endif
 
 	/*
@@ -602,6 +605,9 @@ fd_close(unsigned fd)
 		 * waiting for other users of the file to drain.  Release
 		 * our reference, and wake up the closer.
 		 */
+#ifndef __HAVE_ATOMIC_AS_MEMBAR
+		membar_release();
+#endif
 		atomic_dec_uint(&ff->ff_refcnt);
 		cv_broadcast(&ff->ff_closing);
 		mutex_exit(&fdp->fd_lock);
@@ -637,7 +643,7 @@ fd_close(unsigned fd)
 	} else {
 		/* Multi threaded. */
 #ifndef __HAVE_ATOMIC_AS_MEMBAR
-		membar_producer();
+		membar_release();
 #endif
 		refcnt = atomic_dec_uint_nv(&ff->ff_refcnt);
 	}
@@ -683,7 +689,13 @@ fd_close(unsigned fd)
 			cv_wait(&ff->ff_closing, &fdp->fd_lock);
 		}
 		atomic_and_uint(&ff->ff_refcnt, ~FR_CLOSING);
+#ifndef __HAVE_ATOMIC_AS_MEMBAR
+		membar_acquire();
+#endif
 	} else {
+#ifndef __HAVE_ATOMIC_AS_MEMBAR
+		membar_acquire();
+#endif
 		/* If no references, there must be no knotes. */
 		KASSERT(SLIST_EMPTY(&ff->ff_knlist));
 	}
@@ -1532,10 +1544,13 @@ fd_free(void)
 	KASSERT(fdp->fd_dtbuiltin.dt_link == NULL);
 
 #ifndef __HAVE_ATOMIC_AS_MEMBAR
-	membar_exit();
+	membar_release();
 #endif
 	if (atomic_dec_uint_nv(&fdp->fd_refcnt) > 0)
 		return;
+#ifndef __HAVE_ATOMIC_AS_MEMBAR
+	membar_acquire();
+#endif
 
 	/*
 	 * Close any files that the process holds open.