powerpc/irq: Protect irq_radix_revmap_lookup against irq_free_virt

The radix-tree code uses call_rcu when freeing internal elements.
We must protect against the elements being freed while we traverse
the tree, even if the returned pointer will still be valid.

While preparing a patch to expand the context in which
irq_radix_revmap_lookup will be called, I realized that the
radix tree was not locked.

When asked

For a normal call_rcu usage, is it allowed to read the structure in
irq_enter / irq_exit, without additional rcu_read_lock? Could an
element freed with call_rcu advance with the cpu still between
irq_enter/irq_exit (and irq_disabled())?

Paul McKenney replied:

Absolutely illegal to do so. OK for call_rcu_sched(), but a
flaming bug for call_rcu().

And thank you very much for finding this!!!

Further analysis:

In the current CONFIG_TREE_RCU implementation. CONFIG_TREE_PREEMPT_RCU
(and CONFIG_TINY_PREEMPT_RCU) uses explicit counters.

These counters are reflected from per-CPU to global in the
scheduling-clock-interrupt handler, so disabling irq does prevent the
grace period from completing. But there are real-time implementations
(such as the one use by the Concurrent guys) where disabling irq
does -not- prevent the grace period from completing.

While an alternative fix would be to switch radix-tree to rcu_sched, I
don't want to audit the other users of radix trees (nor put alternative
freeing in the library). The normal overhead for rcu_read_lock and
unlock are a local counter increment and decrement.

This does not show up in the rcu lockdep because in 2.6.34 commit
2676a58c98 (radix-tree: Disable RCU lockdep checking in radix tree)
deemed it too hard to pass the condition of the protecting lock
to the library.

Signed-off-by: Milton Miller <[e-mail]>
Reviewed-by: Paul E. McKenney <[e-mail]>
Signed-off-by: Benjamin Herrenschmidt <[e-mail]>

arch/powerpc/kernel/irq.c

···
 		return irq_find_mapping(host, hwirq);
 
 	/*
-	 * No rcu_read_lock(ing) needed, the ptr returned can't go under us
-	 * as it's referencing an entry in the static irq_map table.
+	 * The ptr returned references the static global irq_map.
+	 * but freeing an irq can delete nodes along the path to
+	 * do the lookup via call_rcu.
 	 */
+	rcu_read_lock();
 	ptr = radix_tree_lookup(&host->revmap_data.tree, hwirq);
+	rcu_read_unlock();
 
 	/*
 	 * If found in radix tree, then fine.