On Thu, Sep 05, 2013 at 11:44:37AM +0200, Miklos Szeredi wrote:
> +static bool __has_unlinked_ancestor(struct dentry *dentry)
> +{
> + struct dentry *this;
> +
> + for (this = dentry; !IS_ROOT(this); this = this->d_parent) {
> + int is_unhashed;
> +
> + /* Need exclusion wrt. check_submounts_and_drop() */
> + spin_lock(&this->d_lock);
> + is_unhashed = d_unhashed(this);
> + spin_unlock(&this->d_lock);
> +
> + if (is_unhashed)
> + return true;
> + }
> + return false;
> +}
> +
> +/*
> + * Called by mount code to check if the mountpoint is reachable (e.g. NFS can
> + * unhash a directory dentry and then the complete subtree can become
> + * unreachable).
> + */
> +bool has_unlinked_ancestor(struct dentry *dentry)
> +{
> + bool found;
> +
> + /* Need exclusion wrt. check_submounts_and_drop() */
> + write_seqlock(&rename_lock);
> + found = __has_unlinked_ancestor(dentry);
> + write_sequnlock(&rename_lock);
> +
> + return found;
> +}
> +
> /*
> * Search the dentry child list of the specified parent,
> * and move any unused dentries to the end of the unused
> diff --git a/fs/internal.h b/fs/internal.h
> index 7c5f01c..d232355 100644
> --- a/fs/internal.h
> +++ b/fs/internal.h
> @@ -126,6 +126,7 @@ extern int invalidate_inodes(struct super_block *, bool);
> * dcache.c
> */
> extern struct dentry *__d_alloc(struct super_block *, const struct qstr *);
> +extern bool has_unlinked_ancestor(struct dentry *dentry);
>
> /*
> * read_write.c
> diff --git a/fs/namespace.c b/fs/namespace.c
> index a45ba4f..91b1c39 100644
> --- a/fs/namespace.c
> +++ b/fs/namespace.c
> @@ -634,6 +634,15 @@ static struct mountpoint *new_mountpoint(struct dentry *dentry)
> }
> dentry->d_flags |= DCACHE_MOUNTED;
> spin_unlock(&dentry->d_lock);
> +
> + if (has_unlinked_ancestor(dentry)) {
> + spin_lock(&dentry->d_lock);
> + dentry->d_flags &= ~DCACHE_MOUNTED;
> + spin_unlock(&dentry->d_lock);
> + kfree(mp);
> + return ERR_PTR(-ENOENT);
> + }
Something's really odd with locking here. You are take d_lock, do one
check, set flag, drop d_lock, grab rename_lock, do another check (taking
and dropping d_lock in process), and, in case that check fails, grab
d_lock again to clear the flag.
At the very least it's a massive overkill. Just grab rename_lock, then
d_lock, then do the damn check and set the flag only on success. Moreover,
with rename_lock held, do you need d_lock on ancestors to mess with in
has_unlinked_ancestor()?
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