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Date: Wed, 26 Nov 2025 10:11:30 +0000
From: Simon Horman <horms@kernel.org>
To: Bobby Eshleman <bobbyeshleman@gmail.com>
Cc: "David S. Miller" <davem@davemloft.net>,
	Eric Dumazet <edumazet@google.com>,
	Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
	Kuniyuki Iwashima <kuniyu@google.com>,
	Willem de Bruijn <willemb@google.com>,
	Neal Cardwell <ncardwell@google.com>,
	David Ahern <dsahern@kernel.org>, Arnd Bergmann <arnd@arndb.de>,
	Jonathan Corbet <corbet@lwn.net>,
	Andrew Lunn <andrew+netdev@lunn.ch>, Shuah Khan <shuah@kernel.org>,
	Donald Hunter <donald.hunter@gmail.com>,
	Mina Almasry <almasrymina@google.com>, netdev@vger.kernel.org,
	linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org,
	linux-doc@vger.kernel.org, linux-kselftest@vger.kernel.org,
	Stanislav Fomichev <sdf@fomichev.me>,
	Bobby Eshleman <bobbyeshleman@meta.com>
Subject: Re: [PATCH net-next v7 3/5] net: devmem: implement autorelease token
 management
Message-ID: <aSbSUre5TNChO4Ah@horms.kernel.org>
References: <20251119-scratch-bobbyeshleman-devmem-tcp-token-upstream-v7-0-1abc8467354c@meta.com>
 <20251119-scratch-bobbyeshleman-devmem-tcp-token-upstream-v7-3-1abc8467354c@meta.com>
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In-Reply-To: <20251119-scratch-bobbyeshleman-devmem-tcp-token-upstream-v7-3-1abc8467354c@meta.com>

On Wed, Nov 19, 2025 at 07:37:10PM -0800, Bobby Eshleman wrote:

...

> @@ -292,25 +327,67 @@ net_devmem_bind_dmabuf(struct net_device *dev,

...

> +	/* Enforce system-wide autorelease mode consistency for RX bindings.
> +	 * TX bindings don't use autorelease (always false) since tokens aren't
> +	 * leaked in TX path. Only RX bindings must all have the same
> +	 * autorelease mode, never mixed.
> +	 *
> +	 * We use the xarray's lock to atomically check xa_empty() and toggle
> +	 * the static key, avoiding the race where two new bindings may try to
> +	 * set the static key to different states.
> +	 */
> +	xa_lock(&net_devmem_dmabuf_bindings);
> +
> +	if (direction == DMA_FROM_DEVICE) {
> +		if (!xa_empty(&net_devmem_dmabuf_bindings)) {
> +			bool mode;
> +
> +			mode = static_key_enabled(&tcp_devmem_ar_key);
> +
> +			/* When bindings exist, enforce that the mode does not
> +			 * change.
> +			 */
> +			if (mode != autorelease) {
> +				NL_SET_ERR_MSG_FMT(extack,
> +						   "System already configured with autorelease=%d",
> +						   mode);
> +				err = -EINVAL;
> +				goto err_unlock_xa;
> +			}
> +		} else {
> +			/* First binding sets the mode for all subsequent
> +			 * bindings.
> +			 */
> +			if (autorelease)
> +				static_branch_enable(&tcp_devmem_ar_key);
> +			else
> +				static_branch_disable(&tcp_devmem_ar_key);

Hi Bobby,

This code runs inside xa_lock, which is a spinlock.

But static_branch_enable() and static_branch_disable()
may sleep due to some combination of taking a mutex
and cpu_read_lock.

Flagged by Claude Code with https://github.com/masoncl/review-prompts/

> +		}
> +	}
> +
> +	err = __xa_alloc_cyclic(&net_devmem_dmabuf_bindings, &binding->id,
> +				binding, xa_limit_32b, &id_alloc_next,
> +				GFP_KERNEL);
>  	if (err < 0)
> -		goto err_free_chunks;
> +		goto err_unlock_xa;
> +
> +	xa_unlock(&net_devmem_dmabuf_bindings);
>  
>  	list_add(&binding->list, &priv->bindings);
>  
>  	return binding;
>  
> +err_unlock_xa:
> +	xa_unlock(&net_devmem_dmabuf_bindings);

...