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    SubjectRe: [PATCH] fix-flush_workqueue-vs-cpu_dead-race-update
    On Sun, Jan 07, 2007 at 11:59:57AM -0800, Andrew Morton wrote:
    > > How would this provide a stable access to cpu_online_map in functions
    > > that need to block while accessing it (as flush_workqueue requires)?
    > If a thread simply blocks, that will not permit a cpu plug/unplug to proceed.
    > The thread had to explicitly call try_to_freeze(). CPU plug/unplug will
    > not occur (and cpu_online_map will not change) until every process in the
    > machine has called try_to_freeze()).

    Maybe my misunderstanding of the code, but:

    Looking at the code, it seems to me that try_to_freeze() will be called very
    likely from the signal-delivery path (get_signal_to_deliver()). Isnt
    that correct? If so, consider a thread as below:

    Thread1 :
    for_each_online_cpu() { online_map = 0x1111 at this point }
    kmalloc(); <- blocks

    Can't Thread1 be frozen in the above blocking state w/o it voluntarily
    calling try_to_freeze? If so, online_map can change when it returns
    from kmalloc() ..

    > So the problem which you're referring to will only occur if a workqueue
    > callback function calls try_to_freeze(), which would be mad.
    > Plus flush_workqueue() is on the way out. We're slowly edging towards a
    > working cancel_work() which will only block if the work which you're trying
    > to cancel is presently running. With that, pretty much all the
    > flush_workqueue() calls go away, and all these accidental rarely-occurring
    > deadlocks go away too.

    Fundamentally, I think it is important to give the ability to block
    concurrent hotplug operations from happening ..

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      Last update: 2007-01-08 16:41    [W:0.022 / U:118.736 seconds]
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