Don't take filedesc lock in fdunshare().

We can read refcnt safely and only care if it is equal to 1.

If it could suddenly change from 1 to something bigger the code would be
buggy even in the previous form and transitions from > 1 to 1 are equally racy
and harmless (we copy even though there is no need).

MFC after:	1 week

1 files changed, 6 insertions, 9 deletions

diff --git a/sys/kern/kern_descrip.c b/sys/kern/kern_descrip.c
index 267b317..7e89c22 100644
--- a/sys/kern/kern_descrip.c
+++ b/sys/kern/kern_descrip.c
@@ -1852,17 +1852,14 @@ fdshare(struct filedesc *fdp)
 void
 fdunshare(struct proc *p, struct thread *td)
 {
+	struct filedesc *tmp;
 
-	FILEDESC_XLOCK(p->p_fd);
-	if (p->p_fd->fd_refcnt > 1) {
-		struct filedesc *tmp;
+	if (p->p_fd->fd_refcnt == 1)
+		return;
 
-		FILEDESC_XUNLOCK(p->p_fd);
-		tmp = fdcopy(p->p_fd);
-		fdescfree(td);
-		p->p_fd = tmp;
-	} else
-		FILEDESC_XUNLOCK(p->p_fd);
+	tmp = fdcopy(p->p_fd);
+	fdescfree(td);
+	p->p_fd = tmp;
 }
 
 /*