Ashok Raj <[email protected]> wrote:
>
> On Sat, Nov 05, 2005 at 03:33:04PM -0800, Andrew Morton wrote:
> > Ashok Raj <[email protected]> wrote:
> > >
> > > Now we leave a trace in current->flags indicating current thread already
> > > is under cpucontrol lock held, so we dont attempt to do this another time.
> > >
> > > ..
> > > +#define PF_HOTPLUG_CPU 0x01000000 /* Currently performing CPU hotplug */
> > >
> >
> > It's still hacky - I mean, we could use this trick to avoid recursion onto
> > any lock in the kernel whenever we get ourselves into a mess. We'd gain an
> > awful lot of PF_* flags.
> >
> > So we should still view this as a temporary fix.
> >
> > I don't think I've seen an analysis of the actual deadlock yet. Are you
> > able to provide a stack trace of the offending callpath?
>
> Hi Andrew,
>
> we call the exact same functions in cpufreq during startup and in
> response to cpu hotplug events, to create or destroy
> sysfs entries /sys/devices/system/cpu/cpuX/cpufreq/*. cpufreq_add_dev().
>
> problem is cpufreq_set_policy() eventually ends up calling
> __cpufreq_driver_target() during the CPU_ONLINE, and CPU_DOWN_PREPARE
> that takes cpucontrol lock.
>
> Since when we already in the cpu notifier callbacks, cpucontrol is already
> held by the cpu_up() or the cpu_down() that caused the double lock.
I appreciate that, but was hoping to see the call graph for the deadlock,
with sysrq-t.
See, one fix might be to take the lock_cpu_hotplug() call out of
__cpufreq_driver_target() and move it higher up the cpufreq call tree.
Into the lowest-level caller which is _above_ the function which the
hotplug code calls.
Like this:
hotplug -> down() -> ... ->
__cpufreq_driver_target()
cpufreq -> down() -> ... ->
and not like this:
hotplug -> down() -> ... ->
__cpufreq_driver_target() -> down()
cpufreq -> ... ->
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