On Fri, Jul 22, 2011 at 08:55:10AM -0400, John Baldwin wrote:
> On Thursday, July 21, 2011 4:19:59 pm Jeremiah Lott wrote:
> > We're seeing nfsclient deadlocks with what looks like lock order reversal
> > after removing a "silly rename". It is fairly rare, but we've seen it
> happen a few times. I included relevant back traces from an occurrence.
> From what I can see, nfs_inactive() is called with the vnode locked. If
> there is a silly-rename, it will call vrele() on its parent directory, which
> can potentially try to lock the parent directory. Since this is the
> opposite order of the lock acquisition in lookup, it can deadlock. This
> happened in a FreeBSD7 build, but I looked through freebsd head and
> didn't see any change that addressed this. Anyone seen this before?
>
> I haven't seen this before, but your analysis looks correct to me.
>
> Perhaps the best fix would be to defer the actual freeing of the sillyrename
> to an asynchronous task? Maybe something like this (untested, uncompiled):
>
> Index: nfsclient/nfsnode.h
> ===================================================================
> --- nfsclient/nfsnode.h (revision 224254)
> +++ nfsclient/nfsnode.h (working copy)
> @@ -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
> @@ -45,8 +46,10 @@
> * can be removed by nfs_inactive()
> */
> struct sillyrename {
> + struct task s_task;
> struct ucred *s_cred;
> struct vnode *s_dvp;
> + struct vnode *s_vp;
> int (*s_removeit)(struct sillyrename *sp);
> long s_namlen;
> char s_name[32];
> Index: nfsclient/nfs_vnops.c
> ===================================================================
> --- nfsclient/nfs_vnops.c (revision 224254)
> +++ nfsclient/nfs_vnops.c (working copy)
> @@ -1757,7 +1757,6 @@
> {
> /*
> * Make sure that the directory vnode is still valid.
> - * XXX we should lock sp->s_dvp here.
> */
> if (sp->s_dvp->v_type == VBAD)
> return (0);
> @@ -2754,8 +2753,10 @@
> M_NFSREQ, M_WAITOK);
> sp->s_cred = crhold(cnp->cn_cred);
> sp->s_dvp = dvp;
> + sp->s_vp = vp;
> sp->s_removeit = nfs_removeit;
> VREF(dvp);
> + vhold(vp);
>
> /*
> * Fudge together a funny name.
> Index: nfsclient/nfs_node.c
> ===================================================================
> --- nfsclient/nfs_node.c (revision 224254)
> +++ nfsclient/nfs_node.c (working copy)
> @@ -47,6 +47,7 @@
> #include <sys/proc.h>
> #include <sys/socket.h>
> #include <sys/sysctl.h>
> +#include <sys/taskqueue.h>
> #include <sys/vnode.h>
>
> #include <vm/uma.h>
> @@ -185,6 +186,26 @@
> return (0);
> }
>
> +static void
> +nfs_freesillyrename(void *arg, int pending)
> +{
> + struct sillyrename *sp;
> +
> + sp = arg;
> + vn_lock(sp->s_dvp, LK_SHARED | LK_RETRY);
I think taking an exclusive lock is somewhat more clean.
> + vn_lock(sp->s_vp, LK_EXCLUSIVE | LK_RETRY);
I believe that you have to verify that at least dvp is not doomed.
Due to this, I propose to only move the vrele() call to taskqueue.
> + (void)nfs_vinvalbuf(ap->a_vp, 0, td, 1);
> + /*
> + * Remove the silly file that was rename'd earlier
> + */
> + (sp->s_removeit)(sp);
> + crfree(sp->s_cred);
> + vput(sp->s_dvp);
> + VOP_UNLOCK(sp->s_vp, 0);
> + vdrop(sp->s_vp);
> + free((caddr_t)sp, M_NFSREQ);
> +}
> +
> int
> nfs_inactive(struct vop_inactive_args *ap)
> {
> @@ -200,15 +221,9 @@
> } else
> sp = NULL;
> if (sp) {
> + TASK_INIT(&sp->task, 0, nfs_freesillyrename, sp);
> + taskqueue_enqueue(taskqueue_thread, &sp->task);
> mtx_unlock(&np->n_mtx);
> - (void)nfs_vinvalbuf(ap->a_vp, 0, td, 1);
> - /*
> - * Remove the silly file that was rename'd earlier
> - */
> - (sp->s_removeit)(sp);
> - crfree(sp->s_cred);
> - vrele(sp->s_dvp);
> - free((caddr_t)sp, M_NFSREQ);
> mtx_lock(&np->n_mtx);
> }
> np->n_flag &= NMODIFIED;
>
> --
> John Baldwin
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