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SubjectRe: Q: NFSD readdir in linux-2.6.28
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On Fri, 2009-03-20 at 00:34 +0900, hooanon05@yahoo.co.jp wrote:
> David Woodhouse:
> > Yes, well spotted. It didn't matter when the buffered readdir() was
> > purely internal to XFS, because it didn't matter there that we called
> > ->lookup() without i_mutex set. But now we're exposing arbitrary file
> > systems to it, we need to make sure we follow the locking rules.
> >
> > I _think_ it's sufficient to make the affected callers of
> > lookup_one_len() lock the parent's i_mutex for themselves before calling
> > it. I'll take a closer look...
>
> If you remember why you discarded the FS_NO_LOOKUP_IN_READDIR flag
> approach, please let me know. URL or something is enough.

I think someone talked me into doing it in the interest of simplicity,
so we confine the necessary hack into the NFS code alone and _always_
just use the "safe" version. I can't remember who it was, but I'm
guessing Al or Christoph -- both of whom are CC'd in case they want to
object:

Given the fun with i_mutex I think the best answer is to bring it back.
I'm about to test this patch which restores it (and sets it for btrfs,
jffs2 and xfs)...

diff --git a/Documentation/filesystems/vfs.txt b/Documentation/filesystems/vfs.txt
index deeeed0..06f539c 100644
--- a/Documentation/filesystems/vfs.txt
+++ b/Documentation/filesystems/vfs.txt
@@ -357,7 +357,10 @@ otherwise noted.
If you wish to overload the dentry methods then you should
initialise the "d_dop" field in the dentry; this is a pointer
to a struct "dentry_operations".
- This method is called with the directory inode semaphore held
+ This method is called with the directory inode mutex held
+ unless the file system sets the FS_NO_LOOKUP_IN_READDIR flag
+ its file system flags, in which case lookup() calls from NFS
+ readdirplus() requests will be made without directory mutex.

link: called by the link(2) system call. Only required if you want
to support hard links. You will probably need to call
diff --git a/fs/btrfs/super.c b/fs/btrfs/super.c
index 9744af9..f0e9d18 100644
--- a/fs/btrfs/super.c
+++ b/fs/btrfs/super.c
@@ -619,7 +619,7 @@ static struct file_system_type btrfs_fs_type = {
.name = "btrfs",
.get_sb = btrfs_get_sb,
.kill_sb = kill_anon_super,
- .fs_flags = FS_REQUIRES_DEV,
+ .fs_flags = FS_REQUIRES_DEV | FS_NO_LOOKUP_IN_READDIR,
};

/*
diff --git a/fs/jffs2/super.c b/fs/jffs2/super.c
index 4c4e18c..0e97a35 100644
--- a/fs/jffs2/super.c
+++ b/fs/jffs2/super.c
@@ -209,6 +209,7 @@ static struct file_system_type jffs2_fs_type = {
.name = "jffs2",
.get_sb = jffs2_get_sb,
.kill_sb = jffs2_kill_sb,
+ .flags = FS_NO_LOOKUP_IN_READDIR,
};

static int __init init_jffs2_fs(void)
diff --git a/fs/nfsd/vfs.c b/fs/nfsd/vfs.c
index ab93fcf..230e30e 100644
--- a/fs/nfsd/vfs.c
+++ b/fs/nfsd/vfs.c
@@ -1841,6 +1841,30 @@ out:
return err;
}

+
+static int nfsd_real_readdir(struct file *file, filldir_t func,
+ struct readdir_cd *cdp, loff_t *offsetp)
+{
+ int host_err;
+
+ /*
+ * Read the directory entries. This silly loop is necessary because
+ * readdir() is not guaranteed to fill up the entire buffer, but
+ * may choose to do less.
+ */
+ do {
+ cdp->err = nfserr_eof; /* will be cleared on successful read */
+ host_err = vfs_readdir(file, func, cdp);
+ } while (host_err >=0 && cdp->err == nfs_ok);
+
+ *offsetp = vfs_llseek(file, 0, 1);
+
+ if (host_err)
+ return nfserrno(host_err);
+ else
+ return cdp->err;
+}
+
/*
* We do this buffering because we must not call back into the file
* system's ->lookup() method from the filldir callback. That may well
@@ -1970,7 +1994,11 @@ nfsd_readdir(struct svc_rqst *rqstp, struct svc_fh *fhp, loff_t *offsetp,
goto out_close;
}

- err = nfsd_buffered_readdir(file, func, cdp, offsetp);
+ if ((file->f_path.dentry->d_inode->i_sb->s_type->fs_flags &
+ FS_NO_LOOKUP_IN_READDIR))
+ err = nfsd_buffered_readdir(file, func, cdp, offsetp);
+ else
+ err = nfsd_real_readdir(file, func, cdp, offsetp);

if (err == nfserr_eof || err == nfserr_toosmall)
err = nfs_ok; /* can still be found in ->err */
diff --git a/fs/xfs/linux-2.6/xfs_super.c b/fs/xfs/linux-2.6/xfs_super.c
index bb68526..7bba46a 100644
--- a/fs/xfs/linux-2.6/xfs_super.c
+++ b/fs/xfs/linux-2.6/xfs_super.c
@@ -1546,7 +1546,7 @@ static struct file_system_type xfs_fs_type = {
.name = "xfs",
.get_sb = xfs_fs_get_sb,
.kill_sb = kill_block_super,
- .fs_flags = FS_REQUIRES_DEV,
+ .fs_flags = FS_REQUIRES_DEV | FS_NO_LOOKUP_IN_READDIR,
};

STATIC int __init
diff --git a/include/linux/fs.h b/include/linux/fs.h
index e766be0..caf2a94 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -176,6 +176,11 @@ struct inodes_stat_t {
#define FS_RENAME_DOES_D_MOVE 32768 /* FS will handle d_move()
* during rename() internally.
*/
+#define FS_NO_LOOKUP_IN_READDIR 65536 /* FS has its own locking and will
+ deadlock if you call its lookup() method from within a filldir
+ function, as NFSd wants to. A file system setting this flag must
+ be happy for its ->lookup() method to be called without i_mutex,
+ which is what the NFSd workaround code will do. */

/*
* These are the fs-independent mount-flags: up to 32 flags are supported

--
dwmw2



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