clk: Evict unregistered clks from parent caches

clk: Evict unregistered clks from parent caches

We leave a dangling pointer in each clk_core::parents array that has an
unregistered clk as a potential parent when that clk_core pointer is
freed by clk{_hw}_unregister(). It is impossible for the true parent of
a clk to be set with clk_set_parent() once the dangling pointer is left
in the cache because we compare parent pointers in
clk_fetch_parent_index() instead of checking for a matching clk name or
clk_hw pointer.

Before commit 7f5f4c15d0d5 ("clk: Remove global clk traversal on fetch
parent index"), we would check clk_hw pointers, which has a higher
chance of being the same between registration and unregistration, but it
can still be allocated and freed by the clk provider. In fact, this has
been a long standing problem since commit 42d611dc519f ("clk: Correct
lookup logic in clk_fetch_parent_index()") where we stopped trying to
compare clk names and skipped over entries in the cache that weren't
NULL.

There are good (performance) reasons to not do the global tree lookup in
cases where the cache holds dangling pointers to parents that have been
unregistered. Let's take the performance hit on the uncommon
registration path instead. Loop through all the clk_core::parents arrays
when a clk is unregistered and set the entry to NULL when the parent
cache entry and clk being unregistered are the same pointer. This will
fix this problem and avoid the overhead for the "normal" case.

Based on a patch by Bjorn Andersson.

Fixes: 42d611dc519f ("clk: Correct lookup logic in clk_fetch_parent_index()")
Reviewed-by: Bjorn Andersson <bjorn.andersson@linaro.org>
Tested-by: Sai Prakash Ranjan <saiprakash.ranjan@codeaurora.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>
Link: https://lkml.kernel.org/r/20190828181959.204401-1-sboyd@kernel.org