Re: [rfc][possible solution] RCU vfsmounts

From: Al Viro
Date: Sun Sep 29 2013 - 14:49:31 EST

On Sun, Sep 29, 2013 at 07:10:47PM +0100, Al Viro wrote:
> FWIW, right now I'm reviewing the subset of fs code that can be hit in
> RCU mode. Not a pretty sight, that... ;-/ First catch: in
> fuse_dentry_revalidate() we have a case (reachable with LOOKUP_RCU) where
> we do this:
> } else if (inode) {
> fc = get_fuse_conn(inode);
> if (fc->readdirplus_auto) {
> parent = dget_parent(entry);
> fuse_advise_use_readdirplus(parent->d_inode);
> dput(parent);
> }
> }
> First of all, that'll lead to obvious nastiness if we get here when
> ->s_fs_info has already been freed in process of fs shutdown; fc will
> be pointing to kfreed object and no, freeing it isn't RCU-delayed.
> That's not a problem with the current tree, of course, but this
> dput(parent) very much is - doing that under rcu_read_lock() is
> a Bloody Bad Idea(tm).

Another one:
int ll_revalidate_nd(struct dentry *dentry, unsigned int flags)
struct inode *parent = dentry->d_parent->d_inode;
int unplug = 0;

CDEBUG(D_VFSTRACE, "VFS Op:name=%s,flags=%u\n",
dentry->, flags);

ll_need_statahead(parent, dentry) > 0) {
if (flags & LOOKUP_RCU)
return -ECHILD;

... and ll_need_statahead(NULL, ...) will oops. Doesn't even need
LOOKUP_RCU to barf - ->d_revalidate() can be called without ->i_mutex
on parent, so we can race with e.g. rename() followed by rmdir() of
old parent.
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