netdev - Re: [PATCH v2 net-next 01/11] af_unix: Define locking order for unix_table_double

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Date: Tue, 11 Jun 2024 19:15:51 -0400
From: Kent Overstreet <kent.overstreet@...ux.dev>
To: Kuniyuki Iwashima <kuniyu@...zon.com>
Cc: "David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Kuniyuki Iwashima <kuni1840@...il.com>, netdev@...r.kernel.org
Subject: Re: [PATCH v2 net-next 01/11] af_unix: Define locking order for
 unix_table_double_lock().

On Tue, Jun 11, 2024 at 03:28:55PM GMT, Kuniyuki Iwashima wrote:
> When created, AF_UNIX socket is put into net->unx.table.buckets[],
> and the hash is stored in sk->sk_hash.
> 
>   * unbound socket  : 0 <= sk_hash <= UNIX_HASH_MOD
> 
> When bind() is called, the socket could be moved to another bucket.
> 
>   * pathname socket : 0 <= sk_hash <= UNIX_HASH_MOD
>   * abstract socket : UNIX_HASH_MOD + 1 <= sk_hash <= UNIX_HASH_MOD * 2 + 1
> 
> Then, we call unix_table_double_lock() which locks a single bucket
> or two.
> 
> Let's define the order as unix_table_lock_cmp_fn() instead of using
> spin_lock_nested().
> 
> The locking is always done in ascending order of sk->sk_hash, which
> is the index of buckets/locks array allocated by kvmalloc_array().
> 
>   sk_hash_A < sk_hash_B
>   <=> &locks[sk_hash_A].dep_map < &locks[sk_hash_B].dep_map
> 
> So, the relation of two sk->sk_hash can be derived from the addresses
> of dep_map in the array of locks.
> 
> Signed-off-by: Kuniyuki Iwashima <kuniyu@...zon.com>

Reviewed-by: Kent Overstreet <kent.overstreet@...ux.dev>

> ---
>  net/unix/af_unix.c | 12 +++++++++++-
>  1 file changed, 11 insertions(+), 1 deletion(-)
> 
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index 3821f8945b1e..22bb941f174e 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -126,6 +126,15 @@ static spinlock_t bsd_socket_locks[UNIX_HASH_SIZE / 2];
>   *    hash table is protected with spinlock.
>   *    each socket state is protected by separate spinlock.
>   */
> +#ifdef CONFIG_PROVE_LOCKING
> +#define cmp_ptr(l, r)	(((l) > (r)) - ((l) < (r)))
> +
> +static int unix_table_lock_cmp_fn(const struct lockdep_map *a,
> +				  const struct lockdep_map *b)
> +{
> +	return cmp_ptr(a, b);
> +}
> +#endif
>  
>  static unsigned int unix_unbound_hash(struct sock *sk)
>  {
> @@ -168,7 +177,7 @@ static void unix_table_double_lock(struct net *net,
>  		swap(hash1, hash2);
>  
>  	spin_lock(&net->unx.table.locks[hash1]);
> -	spin_lock_nested(&net->unx.table.locks[hash2], SINGLE_DEPTH_NESTING);
> +	spin_lock(&net->unx.table.locks[hash2]);
>  }
>  
>  static void unix_table_double_unlock(struct net *net,
> @@ -3578,6 +3587,7 @@ static int __net_init unix_net_init(struct net *net)
>  
>  	for (i = 0; i < UNIX_HASH_SIZE; i++) {
>  		spin_lock_init(&net->unx.table.locks[i]);
> +		lock_set_cmp_fn(&net->unx.table.locks[i], unix_table_lock_cmp_fn, NULL);
>  		INIT_HLIST_HEAD(&net->unx.table.buckets[i]);
>  	}
>  
> -- 
> 2.30.2
>