From 9b7882515864117d0015a3484c0ba0eee6713de9 Mon Sep 17 00:00:00 2001 From: Milton Miller Date: Tue, 24 May 2011 20:34:18 +0000 Subject: [PATCH] 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 Reviewed-by: Paul E. McKenney Signed-off-by: Benjamin Herrenschmidt --- arch/powerpc/kernel/irq.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-) diff --git a/arch/powerpc/kernel/irq.c b/arch/powerpc/kernel/irq.c index ac4d29119f3e..6cb3fcd7fc37 100644 --- a/arch/powerpc/kernel/irq.c +++ b/arch/powerpc/kernel/irq.c @@ -893,10 +893,13 @@ unsigned int irq_radix_revmap_lookup(struct irq_host *host, 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.