Re: [V9fs-developer] [GIT PULL] 9p file system bug fixes for 2.6.35-rc2

Aneesh Kumar K. V
2010-06-24T16:30:20Z

On Wed, 16 Jun 2010 22:12:32 +0530, "Aneesh Kumar K. V" <aneesh.kumar@linux.vnet.ibm.com> wrote:
> On Tue, 8 Jun 2010 01:41:02 +0100, Al Viro <viro@ZenIV.linux.org.uk> wrote:
> > On Mon, Jun 07, 2010 at 05:08:19PM -0700, Linus Torvalds wrote:
> >
> > > In fact, the other thing that I find doing that whole "dentry->d_parent"
> > > thing seems to literally be broken. If you look at v9fs_fid_lookup(),
> > > you'll notice how it walks up the d_parent chain, and at that point you do
> > > NOT own the directory i_mutex, so at that point d_parent really _can_ be
> > > changing wildly due to concurrent renames or whatever.
> >
> > Eh... It's bogus, all right, but i_mutex is not the correct solution.
> > You'd have to take it on a lot of inodes along the way to root *and*
> > you'd violate the ordering in process (ancestors first).
> >
> > I'm not sure what's the right thing to do there, actually - s_vfs_rename_sem
> > also won't do, since it'll give you ordering problems of its own (it's
> > taken before i_mutex in VFS, so trying to take it under i_mutex would not
> > do).
>
> Can we use dcache_lock in v9fs_fid_lookup ? Since we are holding
> parent directory inode->i_mutex in other places where we refer
> dentry->d_parent I guess those access are ok ?. And for v9fs_fid_lookup we
> can hold dcache_lock, walk the parent, build the full path name and use
> that for TWALK ?
>
> Another option is we deny a cross directory rename when
> doing fid_lookup. That is we can introduce a per superblock v9fs
> specific rwlock that get taken (in write mode) in a cross directory
> rename and in fid_lookup we take them in read mode ? We will have to set
> FS_RENAME_DOES_D_MOVE for 9p.

something like this ?

commit 86c06ad7506e7e05dd4e1e1b8cee28e19703c4f6
Author: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>
Date: Mon Jun 21 21:50:07 2010 +0530

fs/9p: Prevent parallel rename when doing fid_lookup

During fid lookup we need to make sure that the dentry->d_parent doesn't
change so that we can safely walk the parent dentries. To ensure that
we need to prevent cross directory rename during fid_lookup. Add a
per superblock rename_lock rwlock to prevent parallel fid lookup and rename.

Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@linux.vnet.ibm.com>

diff --git a/fs/9p/fid.c b/fs/9p/fid.c
index 7317b39..14b542a 100644
--- a/fs/9p/fid.c
+++ b/fs/9p/fid.c
@@ -139,14 +139,19 @@ struct p9_fid *v9fs_fid_lookup(struct dentry *dentry)
fid = v9fs_fid_find(dentry, uid, any);
if (fid)
return fid;
-
+ /*
+ * we don't have a matching fid. To do a TWALK we need
+ * parent fid. We need to prevent rename when we want to
+ * look at the parent.
+ */
+ read_lock(&v9ses->rename_lock);
ds = dentry->d_parent;
fid = v9fs_fid_find(ds, uid, any);
if (!fid) { /* walk from the root */
n = 0;
for (ds = dentry; !IS_ROOT(ds); ds = ds->d_parent)
n++;
-
+ read_unlock(&v9ses->rename_lock);
fid = v9fs_fid_find(ds, uid, any);
if (!fid) { /* the user is not attached to the fs yet */
if (access == V9FS_ACCESS_SINGLE)
@@ -165,9 +170,11 @@ struct p9_fid *v9fs_fid_lookup(struct dentry *dentry)

v9fs_fid_add(ds, fid);
}
- } else /* walk from the parent */
+ } else {
+ /* walk from the parent */
n = 1;
-
+ read_unlock(&v9ses->rename_lock);
+ }
if (ds == dentry)
return fid;

@@ -175,8 +182,10 @@ struct p9_fid *v9fs_fid_lookup(struct dentry *dentry)
if (!wnames)
return ERR_PTR(-ENOMEM);

+ read_lock(&v9ses->rename_lock);
for (d = dentry, i = (n-1); i >= 0; i--, d = d->d_parent)
wnames[i] = (char *) d->d_name.name;
+ read_unlock(&v9ses->rename_lock);

clone = 1;
i = 0;
@@ -199,7 +208,6 @@ struct p9_fid *v9fs_fid_lookup(struct dentry *dentry)
i += l;
clone = 0;
}
-
kfree(wnames);
v9fs_fid_add(dentry, fid);
return fid;
diff --git a/fs/9p/v9fs.c b/fs/9p/v9fs.c
index f8b86e9..6839fca 100644
--- a/fs/9p/v9fs.c
+++ b/fs/9p/v9fs.c
@@ -237,6 +237,7 @@ struct p9_fid *v9fs_session_init(struct v9fs_session_info *v9ses,
__putname(v9ses->uname);
return ERR_PTR(-ENOMEM);
}
+ rwlock_init(&v9ses->rename_lock);

rc = bdi_setup_and_register(&v9ses->bdi, "9p", BDI_CAP_MAP_COPY);
if (rc) {
diff --git a/fs/9p/v9fs.h b/fs/9p/v9fs.h
index bec4d0b..dee4f26 100644
--- a/fs/9p/v9fs.h
+++ b/fs/9p/v9fs.h
@@ -104,6 +104,7 @@ struct v9fs_session_info {
struct p9_client *clnt; /* 9p client */
struct list_head slist; /* list of sessions registered with v9fs */
struct backing_dev_info bdi;
+ rwlock_t rename_lock;
};

struct p9_fid *v9fs_session_init(struct v9fs_session_info *, const char *,
diff --git a/fs/9p/vfs_inode.c b/fs/9p/vfs_inode.c
index 4331b3b..be39035 100644
--- a/fs/9p/vfs_inode.c
+++ b/fs/9p/vfs_inode.c
@@ -678,6 +678,7 @@ static struct dentry *v9fs_vfs_lookup(struct inode *dir, struct dentry *dentry,

sb = dir->i_sb;
v9ses = v9fs_inode2v9ses(dir);
+ /* We can walk d_parent because we hold the dir->i_mutex */
dfid = v9fs_fid_lookup(dentry->d_parent);
if (IS_ERR(dfid))
return ERR_CAST(dfid);
@@ -763,7 +764,7 @@ v9fs_vfs_rename(struct inode *old_dir, struct dentry *old_dentry,
struct p9_fid *olddirfid;
struct p9_fid *newdirfid;
struct p9_wstat wstat;
- int retval;
+ int retval, cross_dir_rename;

P9_DPRINTK(P9_DEBUG_VFS, "\n");
retval = 0;
@@ -784,7 +785,7 @@ v9fs_vfs_rename(struct inode *old_dir, struct dentry *old_dentry,
retval = PTR_ERR(newdirfid);
goto clunk_olddir;
}
-
+ cross_dir_rename = (old_dentry->d_parent != new_dentry->d_parent);
if (v9fs_proto_dotl(v9ses)) {
retval = p9_client_rename(oldfid, newdirfid,
(char *) new_dentry->d_name.name);
@@ -806,6 +807,11 @@ v9fs_vfs_rename(struct inode *old_dir, struct dentry *old_dentry,
retval = p9_client_wstat(oldfid, &wstat);

clunk_newdir:
+ if (cross_dir_rename)
+ write_lock(&v9ses->rename_lock);
+ d_move(old_dentry, new_dentry);
+ if (cross_dir_rename)
+ write_unlock(&v9ses->rename_lock);
p9_client_clunk(newdirfid);

clunk_olddir:
diff --git a/fs/9p/vfs_super.c b/fs/9p/vfs_super.c
index be74d02..0f6d06d 100644
--- a/fs/9p/vfs_super.c
+++ b/fs/9p/vfs_super.c
@@ -278,4 +278,5 @@ struct file_system_type v9fs_fs_type = {
.get_sb = v9fs_get_sb,
.kill_sb = v9fs_kill_super,
.owner = THIS_MODULE,
+ .fs_flags = FS_RENAME_DOES_D_MOVE,
};