On Fri, Oct 16, 2020 at 11:19:41PM -0400, j...@joelfernandes.org wrote:
> On Fri, Oct 16, 2020 at 09:27:53PM -0400, j...@joelfernandes.org wrote:
> [..]
> > > > + *
> > > > + * Memory barrier is needed after adding to length for the case
> > > > + * where length transitions from 0 -> 1. This is because rcu_barrier()
> > > > + * should never miss an update to the length. So the update to length
> > > > + * has to be seen *before* any modifications to the segmented list.
> > > > Otherwise a
> > > > + * race can happen.
> > > > + * P0 (what P1 sees) P1
> > > > + * queue to list
> > > > + * rcu_barrier sees len as 0
> > > > + * set len = 1.
> > > > + * rcu_barrier does nothing.
> > >
> > > So that would be:
> > >
> > > call_rcu() rcu_barrier()
> > > -- --
> > > WRITE(len, len + 1) l = READ(len)
> > > smp_mb() if (!l)
> > > queue check next CPU...
> > >
> > >
> > > But I still don't see against what it pairs in rcu_barrier.
> >
> > Actually, for the second case maybe a similar reasoning can be applied
> > (control dependency) but I'm unable to come up with a litmus test.
> > In fact, now I'm wondering how is it possible that call_rcu() races with
> > rcu_barrier(). The module should ensure that no more call_rcu() should
> > happen
> > before rcu_barrier() is called.
> >
> > confused
>
> So I made a litmus test to show that smp_mb() is needed also after the update
> to length. Basically, otherwise it is possible the callback will see garbage
> that the module cleanup/unload did.
>
> C rcubarrier+ctrldep
>
> (*
> * Result: Never
> *
> * This litmus test shows that rcu_barrier (P1) prematurely
> * returning by reading len 0 can cause issues if P0 does
> * NOT have a smb_mb() after WRITE_ONCE(len, 1).
> * mod_data == 2 means module was unloaded (so data is garbage).
> *)
>
> { int len = 0; int enq = 0; }
>
> P0(int *len, int *mod_data, int *enq)
> {
> int r0;
>
> WRITE_ONCE(*len, 1);
> smp_mb(); /* Needed! */
> WRITE_ONCE(*enq, 1);
>
> r0 = READ_ONCE(*mod_data);
> }
>
> P1(int *len, int *mod_data, int *enq)
> {
> int r0;
> int r1;
>
> r1 = READ_ONCE(*enq);
>
> // barrier Just for test purpose ("exists" clause) to force the..
> // ..rcu_barrier() to see enq before len
> smp_mb();
> r0 = READ_ONCE(*len);
>
> // implicit memory barrier due to conditional */
> if (r0 == 0)
> WRITE_ONCE(*mod_data, 2);
> }
I'm not sure what scenario P1 refers to in practice, and to what module?
>
> // Did P0 read garbage?
> exists (0:r0=2 /\ 1:r0=0 /\ 1:r1=1)
>
What also scares me is that in rcu_barrier():
for_each_possible_cpu(cpu) {
rdp = per_cpu_ptr(&rcu_data, cpu);
if (cpu_is_offline(cpu) &&
!rcu_segcblist_is_offloaded(&rdp->cblist))
continue;
if (rcu_segcblist_n_cbs(&rdp->cblist) && cpu_online(cpu)) {
rcu_barrier_trace(TPS("OnlineQ"), cpu,
rcu_state.barrier_sequence);
smp_call_function_single(cpu, rcu_barrier_func, (void
*)cpu, 1);
} else if (rcu_segcblist_n_cbs(&rdp->cblist) &&
cpu_is_offline(cpu)) {
rcu_barrier_trace(TPS("OfflineNoCBQ"), cpu,
rcu_state.barrier_sequence);
local_irq_disable();
rcu_barrier_func((void *)cpu);
local_irq_enable();
} else if (cpu_is_offline(cpu)) {
rcu_barrier_trace(TPS("OfflineNoCBNoQ"), cpu,
rcu_state.barrier_sequence);
} else {
rcu_barrier_trace(TPS("OnlineNQ"), cpu,
rcu_state.barrier_sequence);
}
}
I can't find something that makes sure this isn't racy while reading
rcu_segcblist_n_cbs(&rdp->cblist).
I mean what I see sums up to this:
CPU 0 CPU 1
rcu_barrier() call_rcu()/rcu_segcblist_enqueue()
------------ --------
smp_mb();
inc_len();
smp_mb();
queue callback;
for_each_possible_cpu(cpu)
if (!rcu_segcblist_n_cbs(&rdp->cblist))
continue;
It looks possible for rcu_barrier() to believe there is no callback enqueued
and see rcu_segcblist_n_cbs(&rdp->cblist) == 0 here.
I'm very likely missing something obvious somewhere.
