[PATCH RFC] clk: use spinlock for clk_get_rate

[mark.langsdorf@calxeda.com (Mark Langsdorf)]

> From: Shawn Guo <shawn.guo@linaro.org>
>
> A nested locking issue is seen on imx6q (CA9 Quad) as below when cpufreq
> driver is running.  It looks like the issue is caused by a simultaneous
> call to clk_get_rate from two smp_twd threads.
> 
> 1) cpu0_set_target
>      cpufreq_notify_transition
>        ...
>          twd_cpufreq_transition
>            twd_update_frequency
>              clk_get_rate
> 
> 2) do_exit
>      kernel_init
>        smp_prepare_cpus
>          percpu_timer_setup
>            twd_timer_setup
>              clk_get_rate
> 
> The patch "clk: new locking scheme for reentrancy" does not help the
> issue.  I'm not sure if this is an issue that should be fixed in
> smp_twd driver or it's an use case which should be supported by clk API.
> 
> Looking at clk_get_rate() API, it just reads a cached clock rate and
> should be fast enough to hold a spinlock than mutex.  It's a quick
> fix to the issue, but I'm not really sure it's the correct one. That's
> why this is a RFC patch.

I was seeing a similar issue while developing cpufreq support for the
Calxeda Highbank SoC. This patch seems to have resolved the issue. I
don't know if it's correct, but it works for me.

--Mark Langsdorf
Calxeda, Inc.