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Message-ID: <8c9879f5-fcfc-69e5-5803-e8b5b1fabfd0@broadcom.com>
Date:   Mon, 14 Feb 2022 09:45:58 +0100
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Subject: Re: locking in wiphy_apply_custom_regulatory()
To:     Johannes Berg <johannes@sipsolutions.net>
Cc:     linux-wireless <linux-wireless@vger.kernel.org>
References: <6562d8c3-27f6-490a-7732-6c300eb3aa64@broadcom.com>
 <a3d73398fa51b66e77d98cf1e883c72b66d6a3f4.camel@sipsolutions.net>
From:   Arend van Spriel <arend.vanspriel@broadcom.com>
In-Reply-To: <a3d73398fa51b66e77d98cf1e883c72b66d6a3f4.camel@sipsolutions.net>
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On 2/11/2022 11:38 AM, Johannes Berg wrote:
> Hi Arend,
> 
>> I stumbled upon locking in wiphy_apply_custom_regulatory() and the
>> history of commits make it a bit unclear to me if these locks are all
>> really needed.
> 
> Oh how I like the regulatory code ... ;)

Thanks for getting in to this in such detail...

>> beee246951571 added the rcu assignments without any locking.
> 
> Well that was just plain wrong, right?
> 
> You have to have some kind of writer-side locking with RCU.
> 
> I think this probably was assuming we'd be under some locks already?
> 
> In regulatory_set_wiphy_regd() vs. regulatory_set_wiphy_regd_sync() we
> later formalized this.

Yeah. That is clear.

>> 51d62f2f2c501 added the rtnl locking.
> 
> That was the bugfix.

Correct. Just not sure I understand why is has to be RTNL lock.

>> a05829a7222e9 added the wiphy lock.
> 
> Yes that's because now get_wiphy_regdom() can be called as documented:
> 
> /*
>   * Returns the regulatory domain associated with the wiphy.
>   *
>   * Requires any of RTNL, wiphy mutex or RCU protection.
>   */

So would wiphy mutex be sufficient. I guess my question is what is 
protected by these lock in wiphy_apply_custom_regulatory() and is it 
really necessary to have both.

>> My understanding of RCU operations was that locking is only needed for
>> readers,
>>
> 
> No, readers are lockless (just "rcu_read_lock()" which enters a critical
> section, but no locking).

Understood. My bad.

>>   but here we use not one but two mutex locks which makes me
>> wonder. Especially, as this function is called before registering the wiphy.
>>
>> Could you clarify?
>>
> 
> Like I said above, that's because we want to dereference the pointer in
> three different contexts:
>   * RCU
>   * RTNL
>   * wiphy mutex
> 
> Maybe we can get rid of some of those readers, but e.g. iwlwifi is using
> 
> 	wiphy_dereference(mvm->hw->wiphy, mvm->hw->wiphy->regd);
> 
> which really should use get_wiphy_regdom(), but I'm not sure is holding
> the RTNL in all paths? Looking at them, it seems like it is, so perhaps
> we can get rid of the wiphy mutex locking.
> 
> 
> Why are you asking? :)

Just some experimental coding where I ended up calling 
wiphy_apply_custom_regulatory() upon IFF_UP and hit deadlock because 
RTNL was already taken. Anyway, that code already ended up in the 
garbage bin, but wanted to ask anyway. Learning by asking (stupid) 
questions ;-)

Regards,
Arend

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