linux-kernel - Re: [PATCH v6 net-next 1/5] net: add napi

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Date:	Wed, 29 May 2013 21:09:52 +0100
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Eliezer Tamir <eliezer.tamir@...ux.intel.com>
CC:	David Miller <davem@...emloft.net>, <linux-kernel@...r.kernel.org>,
	<netdev@...r.kernel.org>,
	Jesse Brandeburg <jesse.brandeburg@...el.com>,
	"Don Skidmore" <donald.c.skidmore@...el.com>,
	<e1000-devel@...ts.sourceforge.net>,
	Willem de Bruijn <willemb@...gle.com>,
	Eric Dumazet <erdnetdev@...il.com>,
	Andi Kleen <andi@...stfloor.org>, HPA <hpa@...or.com>,
	Eilon Greenstien <eilong@...adcom.com>,
	Or Gerlitz <or.gerlitz@...il.com>,
	Alex Rosenbaum <alexr@...lanox.com>,
	Eliezer Tamir <eliezer@...ir.org.il>
Subject: Re: [PATCH v6 net-next 1/5] net: add napi_id and hash

On Wed, 2013-05-29 at 09:39 +0300, Eliezer Tamir wrote:
> Adds a napi_id and a hashing mechanism to lookup a napi by id.
> This will be used by subsequent patches to implement low latency
> Ethernet device polling.
> Based on a code sample by Eric Dumazet.
> 
> Signed-off-by: Eliezer Tamir <eliezer.tamir@...ux.intel.com>
[...]
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
[...]
> @@ -4136,6 +4143,53 @@ void napi_complete(struct napi_struct *n)
>  }
>  EXPORT_SYMBOL(napi_complete);
>  
> +void napi_hash_add(struct napi_struct *napi)
> +{
> +	if (!test_and_set_bit(NAPI_STATE_HASHED, &napi->state)) {
> +
> +		spin_lock(&napi_hash_lock);
> +
> +		/* 0 is not a valid id */
> +		napi->napi_id = 0;
> +		while (!napi->napi_id)
> +			napi->napi_id = ++napi_gen_id;

Suppose we're loading/unloading one driver repeatedly while another one
remains loaded the whole time.  Then once napi_gen_id wraps around, the
same ID can be assigned to multiple contexts.

So far as I can see, assigning the same ID twice will just make polling
stop working for one of the NAPI contexts; I don't think it causes a
crash.  And it is exceedingly unlikely to happen in production.  But if
you're going to the trouble of handling wrap-around at all, you'd better
handle this.

[...]
> +/* must be called under rcu_read_lock(), as we dont take a reference */
> +struct napi_struct *napi_by_id(int napi_id)
> +{
> +	unsigned int hash = napi_id % HASH_SIZE(napi_hash);
[...]

napi_id should be declared unsigned int here, as elsewhere.  The
division can't actually yield a negative result because HASH_SIZE() has
type size_t and napi_id is promoted to match, but I had to go and look
at hashtable.h to check that.

Ben.

-- 
Ben Hutchings, Staff Engineer, Solarflare
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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