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    SubjectRe: Fw: 2.6.16 crashes when running numastat on p575
    Christoph Lameter <> wrote:
    > On Mon, 3 Apr 2006, Nathan Lynch wrote:
    > > > There are many other for_each_*_cpu loops in the kernel that do not have
    > > > any of the instrumentation you suggest. I suggest you come up with a
    > > > general solution and then go through all of them and fix this. Please be
    > > > aware that many of these loops are performance critical.
    > >
    > > But this one isn't, right?
    > Right. One could use more expensive processing here.

    Hopefully none of the for_each_foo() loops are performance-critical - those
    things are expensive.

    > > And I'm afraid there's a misunderstanding here -- only
    > > for_each_online_cpu (or accessing the cpu online map in general) has
    > > such restrictions -- for_each_possible_cpu doesn't require any locking
    > > or preempt tricks since cpu_possible_map must not change after boot.

    for_each_present_cpu() presumably has the same problems.

    > Correct. We may want to audit the kernel and check that each
    > for_each_possible_cpu or for_each_cpu is really correct.

    A fair bit of that has been happening in recent weeks.

    But yes, we should be protecting these things with rcu_read_lock() if
    possible, lock_cpu_hotplug() otherwise.

    (rcu_read_lock() might not be the appropriate name for this operation -
    maybe it should be an open-coded preempt_disable(). Or some other suitably
    named alias; dunno).

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