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, 22 insertions, 2 deletions

diff --git a/sys/fs/nfsclient/nfs_clnode.c b/sys/fs/nfsclient/nfs_clnode.c
index d15de6e..5e7185d 100644
--- a/sys/fs/nfsclient/nfs_clnode.c
+++ b/sys/fs/nfsclient/nfs_clnode.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>
@@ -65,6 +66,8 @@ MALLOC_DECLARE(M_NEWNFSREQ);
 
 uma_zone_t newnfsnode_zone;
 
+static void	nfs_freesillyrename(void *arg, __unused int pending);
+
 void
 ncl_nhinit(void)
 {
@@ -186,6 +189,20 @@ ncl_nget(struct mount *mntp, u_int8_t *fhp, int fhsize, struct nfsnode **npp,
 	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_NEWNFSREQ);
+}
+
 int
 ncl_inactive(struct vop_inactive_args *ap)
 {
@@ -220,8 +237,8 @@ ncl_inactive(struct vop_inactive_args *ap)
 		 */
 		ncl_removeit(sp, vp);
 		crfree(sp->s_cred);
-		vrele(sp->s_dvp);
-		FREE((caddr_t)sp, M_NEWNFSREQ);
+		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/fs/nfsclient/nfsnode.h b/sys/fs/nfsclient/nfsnode.h
index 1d1c89c..c29805d 100644
--- a/sys/fs/nfsclient/nfsnode.h
+++ b/sys/fs/nfsclient/nfsnode.h
@@ -35,11 +35,14 @@
 #ifndef _NFSCLIENT_NFSNODE_H_
 #define	_NFSCLIENT_NFSNODE_H_
 
+#include <sys/_task.h>
+
 /*
  * Silly rename structure that hangs off the nfsnode until the name
  * can be removed by nfs_inactive()
  */
 struct sillyrename {
+	struct	task s_task;
 	struct	ucred *s_cred;
 	struct	vnode *s_dvp;
 	long	s_namlen;