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    SubjectRe: [PATCH 2/2] cpuhotplug: introduce try_get_online_cpus()
    On Mon, Jun 01, 2009 at 09:19:31AM -0700, Paul E. McKenney wrote:
    > On Mon, Jun 01, 2009 at 05:01:50PM +0930, Rusty Russell wrote:
    > > On Sat, 30 May 2009 06:01:18 am Andrew Morton wrote:
    > > > I do think that we should look at
    > > > alternative (non-trylocky) ways of fixing them.
    > >
    > > Speculating: we could add a "keep_cpu()" (FIXME: improve name) which is kind
    > > of like get_cpu() only doesn't disable preemption and only stops *this* cpu
    > > from going down.
    > >
    > > Not sure where that gets us, but if someone's going to dig deep into this it
    > > might help.
    > I have been beating up on the approach of disabling preemption to pin down
    > a single CPU, and although it is working, it is no faster than simply
    > doing get_online_cpus() and it is much much more subtle and complex.
    > I am not sure that I have all the races properly accounted for, and I
    > am failing to see the point of having something quite this ugly in the
    > kernel when much simpler alternatives exist.
    > The main vulnerability is the possibility that someone will invoke
    > synchroniize_rcu_expedited() while holding a mutex that is also acquired
    > in a CPU-hotplug notifier, as Lai noted. But this is easily handled
    > given a primitive that will say whether the current CPU is executing in a
    > CPU-hotplug notifier. This primitive is permitted to sometimes mistakenly
    > say that the current CPU is executing in a CPU-hotplug notifier when it
    > is not (as long as it doesn't do so too often), but not vice versa.
    > One way to implement this would be to have such a primitive simply say
    > whether or not a CPU-hotplug operation is currently in effect. Yes, this
    > is racy, but not when it matters -- you cannot possibly exit a CPU-hotplug
    > operation while executing in a CPU-hotplug notifier. For example,
    > the following exported from kernel/cpu.c would work just fine:
    > bool cpu_hotplug_in_progress(void)
    > {
    > return cpu_hotplug.active_writer != NULL;
    > }
    > I believe that we should be OK moving forward with an updated version of
    > even without the deadlock avoidance.
    > Having the deadlock avoidance would be better, of course, so I will use
    > something like the above on the next patch.

    Of course, the above does not actually solve the deadlock, instead
    merely making it less likely to occur. I have absolutely no idea what
    I was thinking!

    Back to try_get_online_cpus().

    Thanx, Paul

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