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    SubjectRe: [PATCH 1/1] sched: prevent divide by zero error in cpu_avg_load_per_task

    * Steven Rostedt <rostedt@goodmis.org> wrote:

    > From: Steven Rostedt <rostedt@goodmis.org>
    >
    > Impact: fix to divide by zero
    >
    > While testing the branch profiler, I hit this crash:
    >
    > divide error: 0000 [#1] PREEMPT SMP
    > [...]
    > Call Trace:
    > <IRQ> <0> [<ffffffff8024fd43>] find_busiest_group+0x3e5/0xcaa
    > [<ffffffff8025da75>] rebalance_domains+0x2da/0xa21

    > The code for cpu_avg_load_per_task has:
    >
    > if (rq->nr_running)
    > rq->avg_load_per_task = rq->load.weight / rq->nr_running;
    >
    > The runqueue lock is not held here, and there is nothing that
    > prevents the rq->nr_running from going to zero after it passes the
    > if condition.
    >
    > The branch profiler simply made the race window bigger.
    >
    > This patch saves off the rq->nr_running to a local variable and uses
    > that for both the condition and the division.

    good catch! Applied to tip/sched/urgent, thanks Steve!

    the rebalancer scans remote runqueues without holding the runqueue
    lock for performance reasons, so nr_running indeed has to be loaded
    into a local variable here.

    I think it could hit anywhere upstream as well, even without the
    branch tracer: depends on the register pressure and GCC's choices for
    reloading that register. If say FRAME_POINTER is enabled and we are
    running with some more agressive optimization or just an older/suckier
    GCC that does a spurious reload, then this might happen too.

    it's not a classic race, it's an SMP bug that will either trigger or
    not trigger at all, depending on compiler behavior.

    Ingo


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