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    SubjectRe: [PATCH] tracing: add trace_event_read_lock()
    Frederic Weisbecker wrote:
    > On Mon, May 18, 2009 at 07:35:34PM +0800, Lai Jiangshan wrote:
    >> I found that there is nothing to protect event_hash in
    >> ftrace_find_event().
    >
    >
    >
    > Actually, rcu protects it, but not enough. We have neither
    > synchronize_rcu() nor rcu_read_lock.

    We have no rcu_read_lock(), RCU can not protects it.

    >
    > So we protect against concurrent hlist accesses.
    > But the event can be removed when a module is unloaded,
    > and that can happen between the time we get the event output
    > callback and the time we actually use it.
    >

    [...]

    > It could be more fine grained.

    I think it's fine-grained enough, write-side(modules loading/unloading)
    is happened rarely. trace_event_read_lock() will not sleep very likely.

    Thoughts?

    > We could have a per event rwsem, and also place the
    > protected read section only in trace_print_entry() which is the only racy window.
    >

    print_trace_line() is the only racy window.
    So I just protect print_trace_line()(except __ftrace_dump())

    I protect loops which call print_trace_line(), it
    reduces invoke-times:

    trace_event_read_lock();
    while (...) {
    ...
    print_trace_line();
    ...
    }
    trace_event_read_unlock();

    Thanks!
    Lai

    > But I'm not sure it's that worthy since event removal is a rare thing.
    >
    > So I guess this patch is fine.
    >
    >
    >
    >




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