locking in wiphy_apply_custom_regulatory()

[Arend van Spriel <arend.vanspriel@xxxxxxxxxxxx>]

Hi Johannes,

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.

beee246951571 added the rcu assignments without any locking.
51d62f2f2c501 added the rtnl locking.
a05829a7222e9 added the wiphy lock.

My understanding of RCU operations was that locking is only needed for 
readers, 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?

Thanks,
Arend

---8<---------------------------------------------------------------
	/*
	 * no point in calling this if it won't have any effect
	 * on your device's supported bands.
	 */
	WARN_ON(!bands_set);
	new_regd = reg_copy_regd(regd);
	if (IS_ERR(new_regd))
		return;

	rtnl_lock();
	wiphy_lock(wiphy);

	tmp = get_wiphy_regdom(wiphy);
	rcu_assign_pointer(wiphy->regd, new_regd);
	rcu_free_regdom(tmp);

	wiphy_unlock(wiphy);
	rtnl_unlock();
}
EXPORT_SYMBOL(wiphy_apply_custom_regulatory);

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