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    SubjectRe: [Lse-tech] [PATCH] cpusets+hotplug+preepmt broken
    pj wrote:
    > Could you explain why this is -- what is the deadlock?

    On further reading, I see it. You're right.

    Deep in the bowels of the hotplug code, when removing a dead cpu, while
    holding the runqueue lock (task_rq_lock), the hotplug code might need to
    walk up the cpuset hierarchy, to find the nearest enclosing cpuset that
    still has online cpus, as part of figuring where to run a task that is
    being kicked off the dead cpu. The runqueue lock is atomic, but getting
    the cpuset_sem (needed to walk up the cpuset hierarchy) can sleep. So
    you need to get the cpuset_sem before calling task_rq_lock, so as to
    avoid the "scheduling while atomic" oops that you reported. Therefore
    the hotplug code, and anyone else calling cpuset_cpus_allowed(), which
    means sched_setaffinity(), needs to first grab cpuset_sem, so that they
    can grab any atomic locks needed, after getting cpuset_sem, not before.

    Acked-by: Paul Jackson <pj@sgi.com>

    --
    I won't rest till it's the best ...
    Programmer, Linux Scalability
    Paul Jackson <pj@engr.sgi.com> 1.650.933.1373, 1.925.600.0401
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