Re: dcache: remove trylock loops (was Re: [BUG] lock_parent() breakage when used from shrink_dentry_list())

[John Ogness <john.ogness@linutronix.de>]

On 2018-03-14, Al Viro <viro@ZenIV.linux.org.uk> wrote:
>>> +	rcu_read_lock();		/* to protect parent */
>>> +	spin_unlock(&dentry->d_lock);
>>> +	parent = READ_ONCE(dentry->d_parent);
>> 
>> The preceeding line should be removed. We already have a "parent"
>> from before we did the most recent trylock().
>
> Nope.  We have parent, yes, but it had been fetched outside of
> rcu_read_lock().  So the object it used to point to might have been
> already freed and we can't do this:
>
>>> +	spin_lock(&parent->d_lock);

When rcu_read_lock() is called, we are still holding dentry->d_lock. At
that point dentry->d_parent cannot have changed and cannot have been
freed. So the parent fetched outside of rcu_read_lock() is also
protected from freeing inside that rcu_read_lock().

> Come to think of that, it might make sense to lift rcu_read_lock() all
> the way out of that sucker.

Agreed.

> Objections?  Below is the incremental I'd fold into that commit:
>
> diff --git a/fs/dcache.c b/fs/dcache.c
> index f0e73c93182b..0d1dac750c0a 100644
> --- a/fs/dcache.c
> +++ b/fs/dcache.c
> @@ -1000,7 +1000,6 @@ static bool shrink_lock_dentry(struct dentry *dentry)
>  
>  	inode = dentry->d_inode;
>  	if (inode && unlikely(!spin_trylock(&inode->i_lock))) {
> -		rcu_read_lock();	/* to protect inode */
>  		spin_unlock(&dentry->d_lock);
>  		spin_lock(&inode->i_lock);
>  		spin_lock(&dentry->d_lock);
> @@ -1009,16 +1008,14 @@ static bool shrink_lock_dentry(struct dentry *dentry)
>  		/* changed inode means that somebody had grabbed it */
>  		if (unlikely(inode != dentry->d_inode))
>  			goto out;
> -		rcu_read_unlock();
>  	}
>  
>  	parent = dentry->d_parent;
> +	/* parent will stay allocated until we drop rcu_read_lock */

I think this comment is not necessary since this function no longer
deals with dropping rcu_read_lock. But if we keep it, it should be added
for the inode above as well.

>  	if (IS_ROOT(dentry) || likely(spin_trylock(&parent->d_lock)))
>  		return true;
>  
> -	rcu_read_lock();		/* to protect parent */
>  	spin_unlock(&dentry->d_lock);
> -	parent = READ_ONCE(dentry->d_parent);
>  	spin_lock(&parent->d_lock);
>  	if (unlikely(parent != dentry->d_parent)) {
>  		spin_unlock(&parent->d_lock);
> @@ -1026,14 +1023,11 @@ static bool shrink_lock_dentry(struct dentry *dentry)
>  		goto out;
>  	}
>  	spin_lock_nested(&dentry->d_lock, DENTRY_D_LOCK_NESTED);
> -	if (likely(!dentry->d_lockref.count)) {
> -		rcu_read_unlock();
> +	if (likely(!dentry->d_lockref.count))
>  		return true;
> -	}
>  	spin_unlock(&parent->d_lock);
>  out:
>  	spin_unlock(&inode->i_lock);
> -	rcu_read_unlock();
>  	return false;
>  }
>  
> @@ -1044,8 +1038,10 @@ static void shrink_dentry_list(struct list_head *list)
>  
>  		dentry = list_entry(list->prev, struct dentry, d_lru);
>  		spin_lock(&dentry->d_lock);
> +		rcu_read_lock();
>  		if (!shrink_lock_dentry(dentry)) {
>  			bool can_free = false;
> +			rcu_read_unlock();
>  			d_shrink_del(dentry);
>  			if (dentry->d_lockref.count < 0)
>  				can_free = dentry->d_flags & DCACHE_MAY_FREE;
> @@ -1054,6 +1050,7 @@ static void shrink_dentry_list(struct list_head *list)
>  				dentry_free(dentry);
>  			continue;
>  		}
> +		rcu_read_unlock();
>  		d_shrink_del(dentry);
>  		parent = dentry->d_parent;
>  		__dentry_kill(dentry);

John Ogness