- Use an acquire barrier to increment f_count in fget_unlocked and

   remove the volatile cast.  Describe the reason in detail in a comment.

Discussed with:	bde, jhb

1 files changed, 6 insertions, 2 deletions

diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
index c93d05a..1221825 100644
--- a/sys/kern/kern_descrip.c
+++ b/sys/kern/kern_descrip.c
@@ -2075,9 +2075,13 @@ fget_unlocked(struct filedesc *fdp, int fd)
 		count = fp->f_count;
 		if (count == 0)
 			continue;
-		if (atomic_cmpset_int(&fp->f_count, count, count + 1) != 1)
+		/*
+		 * Use an acquire barrier to prevent caching of fd_ofiles
+		 * so it is refreshed for verification.
+		 */
+		if (atomic_cmpset_acq_int(&fp->f_count, count, count + 1) != 1)
 			continue;
-		if (fp == ((struct file *volatile*)fdp->fd_ofiles)[fd])
+		if (fp == fdp->fd_ofiles[fd])
 			break;
 		fdrop(fp, curthread);
 	}