linux-kernel - Re: [PATCH v2] carl9170: Remove redundant protection check

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Date:	Fri, 22 Aug 2014 16:23:19 -0700
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Christian Lamparter <chunkeey@...glemail.com>
Cc:	Andreea-Cristina Bernat <bernat.ada@...il.com>,
	linville@...driver.com, linux-wireless@...r.kernel.org,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH v2] carl9170: Remove redundant protection check

On Fri, 2014-08-22 at 23:53 +0200, Christian Lamparter wrote:

> The sta_info->agg[tid] check is not needed (for reference, see [0]).
> (There is already a check in mac80211 which prevents the leak of
> sta_info->agg[tid] [1]).
> 
> Regards
> Christian
> 
> [0] <https://lkml.org/lkml/2014/8/20/725>
> [1] <http://lxr.free-electrons.com/source/net/mac80211/agg-tx.c#L583>
> 

Hmpfff... this code is quite confusing. RCU is used both in tricky way
(carl9170_ampdu_gc() is an example) and a talisman (the part you remove)

Why is rcu_assign_pointer(sta_info->agg[tid], tid_info);
done inside the spinlock protected region, I don't know.

If this code relies on external protection, a comment would help its
comprehension for sure.

For example, you could add a 
BUG_ON(rcu_access_pointer(sta_info->agg[tid]));
so that we are sure requirements are not changed in the callers one day.

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