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    SubjectRe: [PATCH] tracing/filters: allow on-the-fly filter switching
    On Tue, Apr 14, 2009 at 11:32:25PM -0500, Tom Zanussi wrote:
    > On Tue, 2009-04-14 at 22:56 +0200, Frederic Weisbecker wrote:
    > > On Mon, Apr 13, 2009 at 03:17:50AM -0500, Tom Zanussi wrote:
    >
    > [...]
    >
    > >
    > >
    > > BTW, this issue might be already present before this patch.
    > > What happens if:
    > >
    > >
    > > T1 T2
    > >
    > > event_filter_read() {
    > > filter_print_preds() {
    > > for (i = 0; i < n_preds; i++) {
    > > pred = preds[i];
    > > event_filter_write() {
    > > filter_disable_preds();
    > > filter_clear_preds() {
    > > kfree(pred->field_name);
    > > field_name = pred->field_name;
    > > // CRASH!!!
    > >
    > >
    > > You need a mutex to protect these two callbacks.
    > > It would also protect concurrent calls to event_filter_write(),
    > > which would result in random.
    > >
    >
    > Yeah, Li Zefan had already fixed this, but it wasn't included in this
    > patch. Looks like he'll be resubmitting that part...
    >
    > >
    > >
    >
    > [...]
    >
    > > Also, is filter_add_pred() supposed to be available for in-kernel
    > > uses by other tracers or something?
    >
    > No, the current callers were the only ones I'd planned on (it's not
    > static because code in trace_events.c needs to call it). But, do you
    > see a use for it by other tracers?



    May be in the future, it's possible that a tracer might want to
    set filters by itself.
    But I don't think it has to be fixed now because there are nothing
    like that for now.

    So, no problem :-)

    Frederic.


    > > If this is planned, the locking could be even deeper than my comments.
    > >
    > > Other than these comments:
    > >
    > > Acked-by: Frederic Weisbecker <fweisbec@gmail.com>
    > >
    > > Thanks!
    > >
    >
    > Thanks for reviewing it!
    >
    > Tom
    >



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