Fix a LOR in the NFS client which could cause a deadlock.

This was reported to the mailing list freebsd-net@freebsd.org
on July 21, 2011 under the subject "LOR with nfsclient sillyrename".
The LOR occurred when nfs_inactive() called vrele(sp->s_dvp)
while holding the vnode lock on the file in s_dvp. This patch
modifies the client so that it performs the vrele(sp->s_dvp)
as a separate task to avoid the LOR. This fix was discussed
with jhb@ and kib@, who both proposed variations of it.

Tested by:	pho, jlott at averesystems.com
Submitted by:	jhb (earlier version)
Reviewed by:	kib
Approved by:	re (kib)
MFC after:	2 weeks

2 files changed, 21 insertions, 2 deletions

diff --git a/sys/nfsclient/nfs_node.c b/sys/nfsclient/nfs_node.c
index 5b43b3d..afe3341 100644
--- a/sys/nfsclient/nfs_node.c
+++ b/sys/nfsclient/nfs_node.c
@@ -47,6 +47,7 @@ __FBSDID("$FreeBSD$");
 #include <sys/proc.h>
 #include <sys/socket.h>
 #include <sys/sysctl.h>
+#include <sys/taskqueue.h>
 #include <sys/vnode.h>
 
 #include <vm/uma.h>
@@ -59,6 +60,8 @@ __FBSDID("$FreeBSD$");
 
 static uma_zone_t nfsnode_zone;
 
+static void	nfs_freesillyrename(void *arg, __unused int pending);
+
 #define TRUE	1
 #define	FALSE	0
 
@@ -185,6 +188,20 @@ nfs_nget(struct mount *mntp, nfsfh_t *fhp, int fhsize, struct nfsnode **npp, int
 	return (0);
 }
 
+/*
+ * Do the vrele(sp->s_dvp) as a separate task in order to avoid a
+ * deadlock because of a LOR when vrele() locks the directory vnode.
+ */
+static void
+nfs_freesillyrename(void *arg, __unused int pending)
+{
+	struct sillyrename *sp;
+
+	sp = arg;
+	vrele(sp->s_dvp);
+	free(sp, M_NFSREQ);
+}
+
 int
 nfs_inactive(struct vop_inactive_args *ap)
 {
@@ -207,8 +224,8 @@ nfs_inactive(struct vop_inactive_args *ap)
 		 */
 		(sp->s_removeit)(sp);
 		crfree(sp->s_cred);
-		vrele(sp->s_dvp);
-		free((caddr_t)sp, M_NFSREQ);
+		TASK_INIT(&sp->s_task, 0, nfs_freesillyrename, sp);
+		taskqueue_enqueue(taskqueue_thread, &sp->s_task);
 		mtx_lock(&np->n_mtx);
 	}
 	np->n_flag &= NMODIFIED;
diff --git a/sys/nfsclient/nfsnode.h b/sys/nfsclient/nfsnode.h
index 19c1c47..8e35fdd 100644
--- a/sys/nfsclient/nfsnode.h
+++ b/sys/nfsclient/nfsnode.h
@@ -36,6 +36,7 @@
 #ifndef _NFSCLIENT_NFSNODE_H_
 #define _NFSCLIENT_NFSNODE_H_
 
+#include <sys/_task.h>
 #if !defined(_NFSCLIENT_NFS_H_) && !defined(_KERNEL)
 #include <nfs/nfs.h>
 #endif
@@ -45,6 +46,7 @@
  * can be removed by nfs_inactive()
  */
 struct sillyrename {
+	struct	task s_task;
 	struct	ucred *s_cred;
 	struct	vnode *s_dvp;
 	int	(*s_removeit)(struct sillyrename *sp);