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    SubjectRe: [PATCH 1/1] Fix: trace sched switch start/stop racy updates
    On Sat, 17 Aug 2019 10:27:39 -0400 (EDT)
    Mathieu Desnoyers <mathieu.desnoyers@efficios.com> wrote:

    > I get your point wrt WRITE_ONCE(): since it's a cache it should not have
    > user-visible effects if a temporary incorrect value is observed. Well in
    > reality, it's not a cache: if the lookup fails, it returns "<...>" instead,
    > so cache lookup failure ends up not providing any useful data in the trace.
    > Let's assume this is a known and documented tracer limitation.

    Note, this is done at every sched switch, for both next and prev tasks.
    And the update is only done at the enabling of a tracepoint (very rare
    occurrence) If it missed it scheduling in, it has a really good chance
    of getting it while scheduling out.

    And 99.999% of my tracing that I do, the tasks scheduling in when
    enabling a tracepoint is not what I even care about, as I enable
    tracing then start what I want to trace.


    >
    > However, wrt READ_ONCE(), things are different. The variable read ends up
    > being used to control various branches in the code, and the compiler could
    > decide to re-fetch the variable (with a different state), and therefore
    > cause _some_ of the branches to be inconsistent. See
    > tracing_record_taskinfo_sched_switch() and tracing_record_taskinfo() @flags
    > parameter.

    I'm more OK with using a READ_ONCE() on the flags so it is consistent.
    But the WRITE_ONCE() is going a bit overboard.

    >
    > AFAIU the current code should not generate any out-of-bound writes in case of
    > re-fetch, but no comment in there documents how fragile this is.

    Which part of the code are you talking about here?

    -- Steve

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