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SubjectRe: [PATCH 1/2] trace_events_filter: use rcu_assign_pointer() when setting ftrace_event_call->filter
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[ 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().

>
> TODO: Add proper __rcu annotation to call->filter and all its users.
>
> Signed-off-by: Tejun Heo <tj@kernel.org>
> ---
> kernel/trace/trace_events_filter.c | 6 +++---
> 1 file changed, 3 insertions(+), 3 deletions(-)
>
> Index: work/kernel/trace/trace_events_filter.c
> ===================================================================
> --- work.orig/kernel/trace/trace_events_filter.c
> +++ work/kernel/trace/trace_events_filter.c
> @@ -1686,7 +1686,7 @@ static int replace_system_preds(struct e
> * replace the filter for the call.
> */
> filter = call->filter;
> - call->filter = filter_item->filter;
> + rcu_assign_pointer(call->filter, filter_item->filter);

We update filter here, and then call synchronize_sched() before we free
the filter_item->filter.

> filter_item->filter = filter;
>
> fail = false;
> @@ -1741,7 +1741,7 @@ int apply_event_filter(struct ftrace_eve
> filter = call->filter;
> if (!filter)
> goto out_unlock;
> - call->filter = NULL;
> + rcu_assign_pointer(call->filter, NULL);
> /* Make sure the filter is not being used */

Again you can see that synchronize_sched() is called here.

> synchronize_sched();
> __free_filter(filter);
> @@ -1782,7 +1782,7 @@ out:
> * string
> */
> tmp = call->filter;
> - call->filter = filter;
> + rcu_assign_pointer(call->filter, filter);

We only call synchronize_sched if call->filter wasn't NULL, because we
are going to free tmp. We need to make sure all users are done with tmp
before we free it.

> if (tmp) {
> /* Make sure the call is done with the filter */
> synchronize_sched();

Thus my question is, do we really need to add the rcu_assign_pointer().
I have no problem if we only do so to document that this is an rcu sched
protected variable. But it should be commented as such.

-- Steve




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