Gleb Natapov 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 */
>>
> This is not what happens. irq_rt is never accessed outside read-side
> critical section.
Sorry, I was generalizing to keep the comments short. I figured it
would be clear what I was actually saying, but realize in retrospect
that I was a little ambiguous.
Yes, irq_rt is not accessed outside the RSCS. However, the entry
pointers stored in the irq_rt->map are, and this is equally problematic
afaict.
In this particular case we seem to never delete entries at run-time once
they are established. Therefore, while perhaps sloppy, its technically
safe to leave them unprotected from this perspective. The issue is more
related to shutdown since a kvm_set_irq() caller could be within the
aforementioned race-region and call entry->set() after the guest is
gone. Or did I miss something?
> Data is copied from irq_rt onto the stack and this copy is accessed
> outside critical section.
As mentioned above, I do not believe this really protect us. And even
if it did, the copy is just a work-around to avoid sleeping within the
standard RCU RSCS, which is what SRCU is designed for. So rather than
inventing an awkward two-phased stack based solution, it's better to
reuse the provided tools, IMO.
To flip it around: Is there any reason why an SRCU would not work here,
and thus we were forced to use something like the stack-copy approach?
Kind Regards,
-Greg
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