netdev - Re: [PATCH net-next v9 06/15] net: homa: create homa_sock.h and homa

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Message-ID: <20250613144240.GK414686@horms.kernel.org>
Date: Fri, 13 Jun 2025 15:42:40 +0100
From: Simon Horman <horms@...nel.org>
To: John Ousterhout <ouster@...stanford.edu>
Cc: netdev@...r.kernel.org, pabeni@...hat.com, edumazet@...gle.com,
	kuba@...nel.org
Subject: Re: [PATCH net-next v9 06/15] net: homa: create homa_sock.h and
 homa_sock.c

On Mon, Jun 09, 2025 at 08:40:39AM -0700, John Ousterhout wrote:
> These files provide functions for managing the state that Homa keeps
> for each open Homa socket.
> 
> Signed-off-by: John Ousterhout <ouster@...stanford.edu>

...

> diff --git a/net/homa/homa_sock.h b/net/homa/homa_sock.h

...

> +/**
> + * struct homa_rpc_bucket - One bucket in a hash table of RPCs.
> + */
> +
> +struct homa_rpc_bucket {
> +	/**
> +	 * @lock: serves as a lock both for this bucket (e.g., when
> +	 * adding and removing RPCs) and also for all of the RPCs in
> +	 * the bucket. Must be held whenever looking up an RPC in
> +	 * this bucket or manipulating an RPC in the bucket. This approach
> +	 * has the following properties:
> +	 * 1. An RPC can be looked up and locked (a common operation) with
> +	 *    a single lock acquisition.
> +	 * 2. Looking up and locking are atomic: there is no window of
> +	 *    vulnerability where someone else could delete an RPC after
> +	 *    it has been looked up and before it has been locked.
> +	 * 3. The lookup mechanism does not use RCU.  This is important because
> +	 *    RPCs are created rapidly and typically live only a few tens of
> +	 *    microseconds.  As of May 2027 RCU introduces a lag of about
> +	 *    25 ms before objects can be deleted; for RPCs this would result
> +	 *    in hundreds or thousands of RPCs accumulating before RCU allows
> +	 *    them to be deleted.
> +	 * This approach has the disadvantage that RPCs within a bucket share
> +	 * locks and thus may not be able to work concurently, but there are

nit: concurrently

> +	 * enough buckets in the table to make such colllisions rare.
> +	 *
> +	 * See "Homa Locking Strategy" in homa_impl.h for more info about
> +	 * locking.
> +	 */
> +	spinlock_t lock __context__(rpc_bucket_lock, 1, 1);

As per my comment on __context__ in reply to another patch in this series.
I am clear on the intent of using __context__ here. And it does not seem
to be a common construct within the Kernel. I suspect it would be best
to remove it.

> +
> +	/**
> +	 * @id: identifier for this bucket, used in error messages etc.
> +	 * It's the index of the bucket within its hash table bucket
> +	 * array, with an additional offset to separate server and
> +	 * client RPCs.
> +	 */
> +	int id;
> +
> +	/** @rpcs: list of RPCs that hash to this bucket. */
> +	struct hlist_head rpcs;
> +};

...