On 05/30, Paul E. McKenney wrote:
>
> But it looks like you need the RCU-sched variant. Please see below for
> an untested patch providing this support. One benefit of this patch
> is that it does not add any bloat to Tiny RCU.
I don't think so, see another email. But perhaps I am totally confused,
please correct me.
Well, actually the first writer (need_sync == T) can use it, but it
does not make sense, I think. Because it calls sync() right after
it observes GP_IDLE and drops the lock, the window is too small.
> ------------------------------------------------------------------------
>
> rcu: Add RCU-sched flavors of get-state and cond-sync
However, to me this patch makes sense anyway. Just I don't think rcu_sync
or percpu_rw_semaphore can use the new helpers.
And. I tried to find other users of get_state/cond_sync. Found
ring_buffer_attach() and it looks obviously buggy?
Again, perhaps I am totally confused, but don't we need to ensure
that we have "synchronize" _between_ list_del() and list_add() ?
IOW. Suppose that ring_buffer_attach() preempts right_after
get_state_synchronize_rcu() and gp completes before spin_lock().
In this case cond_synchronize_rcu() does nothing and we reuse
->rb_entry without waiting for gp in between?
Don't we need the patch below? (it also moves the ->rcu_pending check
under "if (rb)", to make it more readable imo).
Peter?
Oleg.
--- x/kernel/events/core.c
+++ x/kernel/events/core.c
@@ -4310,20 +4310,20 @@ static void ring_buffer_attach(struct pe
WARN_ON_ONCE(event->rcu_pending);
old_rb = event->rb;
- event->rcu_batches = get_state_synchronize_rcu();
- event->rcu_pending = 1;
-
spin_lock_irqsave(&old_rb->event_lock, flags);
list_del_rcu(&event->rb_entry);
spin_unlock_irqrestore(&old_rb->event_lock, flags);
- }
- if (event->rcu_pending && rb) {
- cond_synchronize_rcu(event->rcu_batches);
- event->rcu_pending = 0;
+ event->rcu_batches = get_state_synchronize_rcu();
+ event->rcu_pending = 1;
}
if (rb) {
+ if (event->rcu_pending) {
+ cond_synchronize_rcu(event->rcu_batches);
+ event->rcu_pending = 0;
+ }
+
spin_lock_irqsave(&rb->event_lock, flags);
list_add_rcu(&event->rb_entry, &rb->event_list);
spin_unlock_irqrestore(&rb->event_lock, flags);
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
