Question on rcu_access_pointer, rcu_assign_pointer and locking.

[Ben Greear <greearb@candelatech.com>]

Hello!

I'm trying to better understand some code in net/mac80211/scan.c in order
to track down some memory leaks reported by kmemleak.

My question boils down to this.  Assume we have code similar to this:

spin_lock_bh(&dev->bss_lock);
old = rcu_access_pointer(found->pub.beacon_ies);
rcu_assign_pointer(found->pub.beacon_ies, tmp->pub.beacon_ies);
if (old)
	kfree_rcu((struct cfg80211_bss_ies *)old, rcu_head);
spin_unlock_bh(&dev->bss_lock);

Would it be possible to somehow leak what is assigned to found->pub.beacon_ies,
perhaps because two threads managed to go through this
code within a single RCU period?

I think that if the rcu_assign_pointer logic wasn't 'published'
before a second thread came through this logic it could cause
this leakage?

The actual code I'm curious about is in net/mac80211/scan.c, in
the cfg80211_bss_update method.

Thanks,
Ben

-- 
Ben Greear <greearb@candelatech.com>
Candela Technologies Inc  http://www.candelatech.com