Re: [patch 16/52] fs: dcache RCU for multi-step operaitons

john stultz
2010-06-24T17:26:50Z

On Thu, 2010-06-24 at 13:02 +1000, npiggin@suse.de wrote:
> plain text document attachment (fs-dcache_lock-multi-step.patch)
> The remaining usages for dcache_lock is to allow atomic, multi-step read-side
> operations over the directory tree by excluding modifications to the tree.
> Also, to walk in the leaf->root direction in the tree where we don't have
> a natural d_lock ordering.
>
> This could be accomplished by taking every d_lock, but this would mean a
> huge number of locks and actually gets very tricky.
>
> Solve this instead by using the rename seqlock for multi-step read-side
> operations. Insert operations are not serialised. Delete operations are
> tricky when walking up the directory our parent might have been deleted
> when dropping locks so also need to check and retry for that.
>
> XXX: hmm, we could of course just take the rename lock if there is any worry
> about livelock. Most of these are slow paths.

I'll try to point out exactly the spot I think we were hitting in the
-rt tree (once the dcache_lock is removed).

> @@ -1030,9 +1056,15 @@ EXPORT_SYMBOL(have_submounts);
> */
> static int select_parent(struct dentry * parent)
> {
> - struct dentry *this_parent = parent;
> + struct dentry *this_parent;
> struct list_head *next;
> - int found = 0;
> + unsigned seq;
> + int found;
> +
> +rename_retry:
> + found = 0;
> + this_parent = parent;
> + seq = read_seqbegin(&rename_lock);
>
> spin_lock(&dcache_lock);
> spin_lock(&this_parent->d_lock);
> @@ -1043,7 +1075,6 @@ resume:
> struct list_head *tmp = next;
> struct dentry *dentry = list_entry(tmp, struct dentry, d_u.d_child);
> next = tmp->next;
> - BUG_ON(this_parent == dentry);
>
> spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
> dentry_lru_del_init(dentry);
> @@ -1084,17 +1115,33 @@ resume:
> */
> if (this_parent != parent) {
> struct dentry *tmp;
> - next = this_parent->d_u.d_child.next;
> + struct dentry *child;
> +
> tmp = this_parent->d_parent;
> + rcu_read_lock();
> spin_unlock(&this_parent->d_lock);
> - BUG_ON(tmp == this_parent);
> + child = this_parent;
> this_parent = tmp;

Ok. So right here, we get preempted, or dput() is called by another cpu
on the child dentry, or the child->d_u.d_child.next dentry and its
d_kill'ed.

> spin_lock(&this_parent->d_lock);
> + /* might go back up the wrong parent if we have had a rename
> + * or deletion */
> + if (this_parent != child->d_parent ||
> + // d_unlinked(this_parent) || XXX
> + read_seqretry(&rename_lock, seq)) {
> + spin_unlock(&this_parent->d_lock);
> + spin_unlock(&dcache_lock);
> + rcu_read_unlock();
> + goto rename_retry;
> + }
> + rcu_read_unlock();
> + next = child->d_u.d_child.next;

Then at this point, next may point to junk.

> goto resume;
> }
> out:
> spin_unlock(&this_parent->d_lock);
> spin_unlock(&dcache_lock);
> + if (read_seqretry(&rename_lock, seq))
> + goto rename_retry;
> return found;
> }

thanks
-john