Re: [PATCH v2 3/6] cputlb: ensure we save the IOTLB data in case of reset

[Emilio G. Cota]

On Mon, Jun 22, 2020 at 10:02:50 +0100, Alex Bennée wrote:
> Emilio G. Cota <cota@braap.org> writes:
> > On Wed, Jun 10, 2020 at 16:55:06 +0100, Alex Bennée wrote:
(snip)
> >> +#ifdef CONFIG_PLUGIN
> >> +
> >> +typedef struct SavedIOTLB {
> >> +    struct rcu_head rcu;
> >> +    struct SavedIOTLB **save_loc;
> >> +    MemoryRegionSection *section;
> >> +    hwaddr mr_offset;
> >> +} SavedIOTLB;
> >> +
> >> +static void clean_saved_entry(SavedIOTLB *s)
> >> +{
> >> +    atomic_rcu_set(s->save_loc, NULL);
> >
> > This will race with the CPU thread that sets saved_for_plugin in
> > save_iotlb_data().
> 
> Surely that only happens outside the critical section?

I am not sure which critical section you're referring to.

With call_rcu() we defer the execution of the function to the RCU
thread at a later time, where "later time" is defined as any time
after the pre-existing RCU read critical sections have elapsed.

So we could have the RCU thread clearing the variable while the
CPU thread, which is in a _later_ RCU read critical section, is
setting said variable. This is the race I was referring to.

Thanks,
        
                Emilio