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SubjectRe: [KVM PATCH v3 1/3] KVM: fix race in irq_routing logic
Hi Paul,

Paul E. McKenney wrote:
> On Mon, Oct 26, 2009 at 12:21:57PM -0400, Gregory Haskins wrote:
>> The current code suffers from the following race condition:
>>
>> thread-1 thread-2
>> -----------------------------------------------------------
>>
>> kvm_set_irq() {
>> rcu_read_lock()
>> irq_rt = rcu_dereference(table);
>> rcu_read_unlock();
>>
>> kvm_set_irq_routing() {
>> mutex_lock();
>> irq_rt = table;
>> rcu_assign_pointer();
>> mutex_unlock();
>> synchronize_rcu();
>>
>> kfree(irq_rt);
>>
>> irq_rt->entry->set(); /* bad */
>>
>> -------------------------------------------------------------
>>
>> Because the pointer is accessed outside of the read-side critical
>> section. There are two basic patterns we can use to fix this bug:
>>
>> 1) Switch to sleeping-rcu and encompass the ->set() access within the
>> read-side critical section,
>>
>> OR
>>
>> 2) Add reference counting to the irq_rt structure, and simply acquire
>> the reference from within the RSCS.
>>
>> This patch implements solution (1).
>
> Looks like a good transformation! A few questions interspersed below.

Thanks for the review. I would have CC'd you but I figured I pestered
you enough with my RCU reviews in the past, and didn't want to annoy you ;)

I will be sure to CC you in the future, unless you ask otherwise.

>
>> Signed-off-by: Gregory Haskins <ghaskins@novell.com>
>> ---
>>
>> include/linux/kvm_host.h | 6 +++++-
>> virt/kvm/irq_comm.c | 50 +++++++++++++++++++++++++++-------------------
>> virt/kvm/kvm_main.c | 1 +
>> 3 files changed, 35 insertions(+), 22 deletions(-)
>>
>> diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
>> index bd5a616..1fe135d 100644
>> --- a/include/linux/kvm_host.h
>> +++ b/include/linux/kvm_host.h
>> @@ -185,7 +185,10 @@ struct kvm {
>>
>> struct mutex irq_lock;
>> #ifdef CONFIG_HAVE_KVM_IRQCHIP
>> - struct kvm_irq_routing_table *irq_routing;
>> + struct {
>> + struct srcu_struct srcu;
>
> Each structure has its own SRCU domain. This is OK, but just asking
> if that is the intent. It does look like the SRCU primitives are
> passed a pointer to the correct structure, and that the return value
> from srcu_read_lock() gets passed into the matching srcu_read_unlock()
> like it needs to be, so that is good.

Yeah, it was intentional. Technically the table is per-guest, and thus
the locking is too, which is the desired/intentional granularity.

On that note, I tried to denote that kvm->irq_routing.srcu and
kvm->irq_routing.table were related to one another, but then went ahead
and modified the hunks that touched kvm->irq_ack_notifier_list, too. In
retrospect, this was probably a mistake. I should leave the rcu usage
outside of ->irq_routing.table alone.

>
>> + struct kvm_irq_routing_table *table;
>> + } irq_routing;
>> struct hlist_head mask_notifier_list;
>> struct hlist_head irq_ack_notifier_list;
>> #endif
>
> [ . . . ]
>
>> @@ -155,21 +156,19 @@ int kvm_set_irq(struct kvm *kvm, int irq_source_id, u32 irq, int level)
>> * IOAPIC. So set the bit in both. The guest will ignore
>> * writes to the unused one.
>> */
>> - rcu_read_lock();
>> - irq_rt = rcu_dereference(kvm->irq_routing);
>> + idx = srcu_read_lock(&kvm->irq_routing.srcu);
>> + irq_rt = rcu_dereference(kvm->irq_routing.table);
>> if (irq < irq_rt->nr_rt_entries)
>> - hlist_for_each_entry(e, n, &irq_rt->map[irq], link)
>> - irq_set[i++] = *e;
>> - rcu_read_unlock();
>> + hlist_for_each_entry(e, n, &irq_rt->map[irq], link) {
>
> What prevents the above list from changing while we are traversing it?
> (Yes, presumably whatever was preventing it from changing before this
> patch, but what?)
>
> Mostly kvm->lock is held, but not always. And if kvm->lock were held
> all the time, there would be no point in using SRCU. ;-)

This is protected by kvm->irq_lock within kvm_set_irq_routing().
Entries are added to a copy of the list, and the top-level table pointer
is swapped (via rcu_assign_pointer(), as it should be) while holding the
lock. Finally, we synchronize with the RSCS before deleting the old
copy. It looks to me like the original author got this part right, so I
didn't modify it outside of converting to SRCU.

>
>> + int r;
>>
>> - while(i--) {
>> - int r;
>> - r = irq_set[i].set(&irq_set[i], kvm, irq_source_id, level);
>> - if (r < 0)
>> - continue;
>> + r = e->set(e, kvm, irq_source_id, level);
>> + if (r < 0)
>> + continue;
>>
>> - ret = r + ((ret < 0) ? 0 : ret);
>> - }
>> + ret = r + ((ret < 0) ? 0 : ret);
>> + }
>> + srcu_read_unlock(&kvm->irq_routing.srcu, idx);
>>
>> return ret;
>> }
>> @@ -179,17 +178,18 @@ void kvm_notify_acked_irq(struct kvm *kvm, unsigned irqchip, unsigned pin)
>> struct kvm_irq_ack_notifier *kian;
>> struct hlist_node *n;
>> int gsi;
>> + int idx;
>>
>> trace_kvm_ack_irq(irqchip, pin);
>>
>> - rcu_read_lock();
>> - gsi = rcu_dereference(kvm->irq_routing)->chip[irqchip][pin];
>> + idx = srcu_read_lock(&kvm->irq_routing.srcu);
>> + gsi = rcu_dereference(kvm->irq_routing.table)->chip[irqchip][pin];
>> if (gsi != -1)
>> hlist_for_each_entry_rcu(kian, n, &kvm->irq_ack_notifier_list,
>> link)
>
> And same question here -- what keeps the above list from changing while
> we are traversing it?

This is also protected via the kvm->irq_lock in
kvm_register_irq_ack_notifier(). Though as mentioned above, I should
probably drop the non irq_routing.table hunks, so this will go away.
But I think its correct either way.

Thanks Paul,
-Greg

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