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    SubjectRe: [RFD] Freezing of kernel threads
    Date
    On Monday, 14 May 2007 09:26, Gautham R Shenoy wrote:
    > On Mon, May 14, 2007 at 11:48:46AM +0530, Srivatsa Vaddagiri wrote:
    > >
    > > The other complication get/put_hotcpu() had was dealing with
    > > write-followed-by-read lock attempt by the *same* thread (whilst doing
    > > cpu_down/up). IIRC this was triggered by some callback processing in CPU_DEAD
    > > or CPU_DOWN_PREPARE.
    > >
    > >
    > > cpu_down()
    > > |- take write lock
    > > |- CPU_DOWN_PREPARE
    > > | |- foo() wants a read_lock
    > >
    > > Stupid as it sounds, it was really found to be happening! Gautham, do you
    > > recall who that foo() was? Somebody in cpufreq I guess ..
    >
    > IIRC, it was a problem with ondemand. while handling CPU_DEAD, ondemand code
    > would call destroy_workqueue, which tried flushing the workqueue, which
    > once upon a time did lock_cpu_hotplug, before Oleg and Andrew cleaned
    > that up.
    >
    > Ofcourse, cpufreq works fine now after Venki's patches which
    > just nullifies the reference to the policy structure of the cpu to be
    > removed during the CPU_DOWN_PREPARE by calling __cpufreq_remove_dev
    > instead of handling it in CPU_DEAD.
    >
    > However, as we have discovered, without freezing all the threads, it
    > is inadvisable to call flush_workqueue from a cpu-hotplug callback
    > path.

    Please see my recent patch at http://lkml.org/lkml/2007/5/14/7 .
    It's not exactly the same thing, but I think the trick in there might be
    useful.

    Greetings,
    Rafael
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