Re: BUG fs/dcache.c:595 in 2.4.24rc3-git3 during NFS umount

[Trond Myklebust <trond.myklebust@fys.uio.no>]

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On Mon, 2007-12-10 at 19:57 +0100, Andi Kleen wrote:
> > Hi Andi,
> > 
> > Does the attached patch fix the Oops?
> 
> Nope, got that a few hours after boot again:
> 
> -Andi

Hi Andi,

I appear to have misread d_find_alias(). It would seem that the only way
to ensure that a mountpoint won't be found is to remove it altogether
from the inode->i_dentry list. AFAICS that should be largely harmless
since the nfs sb->s_root is never visible to users, and is never part of
a dentry tree.

Cheers
  Trond


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From: Trond Myklebust <Trond.Myklebust@netapp.com>
Subject: NFS: Fix an Oops in NFS unmount
Date: 
Message-ID: <1197428085.28126.4.camel@heimdal.trondhjem.org>

Ensure that the dummy 'root dentry' is invisible to d_find_alias(). If not,
then it may be spliced into the tree if a parent directory from the same
filesystem gets mounted at a later time.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>
---

 fs/nfs/getroot.c |   11 +++++++++++
 1 files changed, 11 insertions(+), 0 deletions(-)

diff --git a/fs/nfs/getroot.c b/fs/nfs/getroot.c
index 0ee4384..e6242cd 100644
--- a/fs/nfs/getroot.c
+++ b/fs/nfs/getroot.c
@@ -57,6 +57,17 @@ static int nfs_superblock_set_dummy_root(struct super_block *sb, struct inode *i
 		}
 		/* Circumvent igrab(): we know the inode is not being freed */
 		atomic_inc(&inode->i_count);
+		/*
+		 * Ensure that this dentry is invisible to d_find_alias().
+		 * Otherwise, it may be spliced into the tree by
+		 * d_materialise_unique if a parent directory from the same
+		 * filesystem gets mounted at a later time.
+		 * This again causes shrink_dcache_for_umount_subtree() to
+		 * Oops, since the test for IS_ROOT() will fail.
+		 */
+		spin_lock(&dcache_lock);
+		list_del_init(&sb->s_root->d_alias);
+		spin_unlock(&dcache_lock);
 	}
 	return 0;
 }