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    SubjectRe: [PATCH 02/10] perf, trace: Use per-tracepoint-per-cpu hlist to track events
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    On Fri, 2010-05-21 at 12:21 +0200, Frederic Weisbecker wrote:
    > On Fri, May 21, 2010 at 12:19:09PM +0200, Peter Zijlstra wrote:
    > > On Fri, 2010-05-21 at 12:13 +0200, Frederic Weisbecker wrote:
    > > > > I assumed that after probe unregister a tracepoint callback doesn't
    > > > > happen, which then guarantees we should never get !head.
    > >
    > > > I'm not sure about this. The tracepoints are called under rcu_read_lock(),
    > > > but there is not synchronize_rcu() after we unregister a tracepoint, which
    > > > means you can have a pending preempted one somewhere.
    > > >
    > > > There is a call_rcu that removes the callbacks, but that only protect
    > > > the callback themselves.
    > >
    > > Ah, ok, so we should do probe_unregister + synchronize_sched().
    > > That should ensure __DO_TRACE() doesn't call into it anymore.
    > >
    > > /me goes make a patch
    > >
    >
    >
    > Yep. But that also means we need to rcu_dereference_sched() to access
    > the per cpu list of events.

    Why?

    The per-cpu vars are allocated and freed in a fully serialized manner,
    there should be no races what so ever.



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