Re: Is stopmachine() preempt safe?

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On Mon, 2006-08-28 at 16:36 +1000, Keith Owens wrote:
> There is a lot of code in the kernel that runs cpu_online_map without
> taking any locks and without disabling preemption.  Obviously we do not
> want all that code to lock or disable preemption, it will kill
> scalability.

There is actually not much code which should use cpu_online_map.  Code
which does must be careful: you generally need to think about handling
cpu hotplug notifiers as well as the map changing underneath you.

Doing a brief audit, ignoring the already-acknowledged cpufreq code and
arch-specifics, I can see these cases which seem suspicious:

        - I assume this is relying on some other mechanism so the cpu
        doesn't get onlined?
        - A couple of other num_online_cpus() there in ACPI might need a
        rethink for hotplug CPU though.

./kernel/irq/proc.c:  irq_affinity_write_proc()
        - seems complicated, but I think migration.c handles when cpus
        gone offline?

        - needs to handle hotplug cpus (or just say don't do that?)

        - seems to be using num_online_cpus as a really poor heuristic,
        and incorrectly (for i = 0; i < num_online_cpus(); i++) <-- i is
        not a valid CPU number!).

        - suspicious, code here, too.
./net/core/dev.c: net_dma_rebalance()
        - This is a heuristic, which may be OK.
./net/core/dev.c: softnet_get_online()
        - It'd be nice if net/dev/core.c used cpu_possible() not
        cpu_online() to report stats, so they don't get lost from
        offlined CPUs.

./net/core/pktgen.c: pg_init()
        - Assumes no CPU plugging, but is a pretty specialized driver.
(Other uses get away with being in initcalls, or on platforms without
hotplug CPU).

Disappointingly, none of these would be fixed by changing the semantics
of stop_machine; they rely on the online cpus and must take action when
they change, whether they are reading the online_cpu_map at the time or

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