netdev - Re: [net-next PATCH v2 03/12] net: flow: implement flow cache for get routines

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Date:	Wed, 14 Jan 2015 19:21:33 -0800
From:	John Fastabend <john.fastabend@...il.com>
To:	Thomas Graf <tgraf@...g.ch>
CC:	simon.horman@...ronome.com, sfeldma@...il.com,
	netdev@...r.kernel.org, gerlitz.or@...il.com, jhs@...atatu.com,
	andy@...yhouse.net, davem@...emloft.net
Subject: Re: [net-next PATCH v2 03/12] net: flow: implement flow cache for
 get routines

On 01/14/2015 01:52 PM, Thomas Graf wrote:
> On 01/13/15 at 01:36pm, John Fastabend wrote:
>> I chose rhashtable to get the dynamic resizing. I could use arrays
>> but I don't want to pre-allocate large cache tables when we may
>> never use them.
>>
>> One oddity in the rhashtable implementation is there is no way
>> AFAICS to do delayed free's so we use rcu_sync heavily. This should be
>> fine, get operations shouldn't be a used heavily.
>
> John, can you please clarify a bit, I'm not sure I understand. Are you
> talking about delayed freeing of the table itself or elements?
>
> The Netlink usage would be an example of a user with delayed element
> freeing.
>
> I'm glad to add whatever is required.
>

Took another look at the netlink code looks like this is the correct
pattern where the call_rcu implements the delayed freeing after a grace
period.

	mutex_lock(&my_hash_lock);
	rhashtable_remove(&my_hash, &my_obj->rhash_head);
	mutex_unlock(&my_hash_lock)

	[...]

	call_rcu(&my_obj->rcu, deferred_my_obj_free);

anyways it looks like it is there no problem after all and I don't
recall what I was thinking thanks for bearing with me. I'll convert
this code to avoid the over-use of rcu_sync.

Thanks,
John

-- 
John Fastabend         Intel Corporation
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