Re: [PATCH 1/2] trace_events_filter: use rcu_assign_pointer() whensetting ftrace_event_call->filter
From: Tejun Heo
Date: Wed Nov 23 2011 - 11:07:01 EST
On Wed, Nov 23, 2011 at 10:16:47AM -0500, Steven Rostedt wrote:
> [ Added Paul to Cc ]
>
> On Tue, 2011-11-22 at 17:46 -0800, Tejun Heo wrote:
> > ftrace_event_call->filter is sched RCU protected but didn't use
> > rcu_assign_pointer(). Fix it.
>
> Is it really needed? Maybe just for documentation but I'm not sure this
> use is required because all use cases have synchronize_sched() used,
> which is a big hammer compared to the rcu_assign_pointer().
Oh yeah, I think we do. synchronize_sched() is to drain users of the
old pointer. Whether synchronize_sched() or call_rcu() is used is
irrelevant to the synchronization of new pointer.
rcu_assign_pointer() is to synchronize against rcu_dereference()
dereferencing the new one. More specifically, we need it for the
writer memory barrier, so that it matches the data dependency read
barrier in rcu_dereference() when it access the new pointer;
otherwise, it may fetch the old values from before the new area is
initialized on archs where data dependency barrier isn't noop.
> We update filter here, and then call synchronize_sched() before we free
> the filter_item->filter.
...
> Again you can see that synchronize_sched() is called here.
So, synchronized_sched() being called after isn't relevant. We want
smp_wmb() between data structure initialization and assignment of the
new pointer.
Thank you.
--
tejun
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/