linux-kernel - Re: [PATCH net-next v10 11/14] tcp: RX path for devmem TCP

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Date: Tue, 04 Jun 2024 12:53:35 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Mina Almasry <almasrymina@...gle.com>, netdev@...r.kernel.org, 
	linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org, 
	linux-alpha@...r.kernel.org, linux-mips@...r.kernel.org, 
	linux-parisc@...r.kernel.org, sparclinux@...r.kernel.org, 
	linux-trace-kernel@...r.kernel.org, linux-arch@...r.kernel.org, 
	bpf@...r.kernel.org, linux-kselftest@...r.kernel.org, 
	linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org
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Subject: Re: [PATCH net-next v10 11/14] tcp: RX path for devmem TCP

On Thu, 2024-05-30 at 20:16 +0000, Mina Almasry wrote:
> @@ -2317,6 +2318,213 @@ static int tcp_inq_hint(struct sock *sk)
>  	return inq;
>  }
>  
> +/* batch __xa_alloc() calls and reduce xa_lock()/xa_unlock() overhead. */
> +struct tcp_xa_pool {
> +	u8		max; /* max <= MAX_SKB_FRAGS */
> +	u8		idx; /* idx <= max */
> +	__u32		tokens[MAX_SKB_FRAGS];
> +	netmem_ref	netmems[MAX_SKB_FRAGS];
> +};
> +
> +static void tcp_xa_pool_commit(struct sock *sk, struct tcp_xa_pool *p,
> +			       bool lock)
> +{
> +	int i;
> +
> +	if (!p->max)
> +		return;
> +	if (lock)
> +		xa_lock_bh(&sk->sk_user_frags);

The conditional lock here confuses sparse.

I think you can avoid it providing a unlocked version (no need to check
for '!p->max' the only caller wanting the unlocked version already
performs such check) and a locked one, calling the other.

Cheers,

Paolo