Re: [net-next PATCH 3/3] net: frag queue per hash bucket locking

[Eric Dumazet <eric.dumazet@gmail.com>]

On Wed, 2013-03-27 at 16:56 +0100, Jesper Dangaard Brouer wrote:
> This patch implements per hash bucket locking for the frag queue
> hash.  This removes two write locks, and the only remaining write
> lock is for protecting hash rebuild.  This essentially reduce the
> readers-writer lock to a rebuild lock.
> 

> @@ -226,27 +247,32 @@ static struct inet_frag_queue *inet_frag_intern(struct netns_frags *nf,
>  		struct inet_frag_queue *qp_in, struct inet_frags *f,
>  		void *arg)
>  {
> +	struct inet_frag_bucket *hb;
>  	struct inet_frag_queue *qp;
>  #ifdef CONFIG_SMP
>  #endif
>  	unsigned int hash;
>  
> -	write_lock(&f->lock);
> +	read_lock(&f->lock); /* Protects against hash rebuild */
>  	/*
>  	 * While we stayed w/o the lock other CPU could update
>  	 * the rnd seed, so we need to re-calculate the hash
>  	 * chain. Fortunatelly the qp_in can be used to get one.
>  	 */
>  	hash = f->hashfn(qp_in);
> +	hb = &f->hash[hash];
> +	spin_lock_bh(&hb->chain_lock);
> +
>  #ifdef CONFIG_SMP
>  	/* With SMP race we have to recheck hash table, because
>  	 * such entry could be created on other cpu, while we
> -	 * promoted read lock to write lock.
> +	 * released the hash bucket lock.
>  	 */
> -	hlist_for_each_entry(qp, &f->hash[hash], list) {
> +	hlist_for_each_entry(qp, &hb->chain, list) {
>  		if (qp->net == nf && f->match(qp, arg)) {
>  			atomic_inc(&qp->refcnt);
> -			write_unlock(&f->lock);
> +			spin_unlock_bh(&hb->chain_lock);
> +			read_unlock(&f->lock);
>  			qp_in->last_in |= INET_FRAG_COMPLETE;
>  			inet_frag_put(qp_in, f);
>  			return qp;
> @@ -258,8 +284,10 @@ static struct inet_frag_queue *inet_frag_intern(struct netns_frags *nf,
>  		atomic_inc(&qp->refcnt);
>  
>  	atomic_inc(&qp->refcnt);
> -	hlist_add_head(&qp->list, &f->hash[hash]);
> -	write_unlock(&f->lock);
> +	hlist_add_head(&qp->list, &hb->chain);
> +	hb->chain_len++;
> +	spin_unlock_bh(&hb->chain_lock);
> +	read_unlock(&f->lock);
>  	inet_frag_lru_add(nf, qp);
>  	return qp;
>  }


I am not sure why you added _bh suffix to spin_lock()/spin_unlock()
here ?

(Please check in other functions as well)