[PATCH RFC] clk: use spinlock for clk_get_rate

From: Shawn Guo <redacted>

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.