radix-tree: Use local_lock for protection

radix-tree: Use local_lock for protection

The radix-tree and idr preload mechanisms use preempt_disable() to protect
the complete operation between xxx_preload() and xxx_preload_end().

As the code inside the preempt disabled section acquires regular spinlocks,
which are converted to 'sleeping' spinlocks on a PREEMPT_RT kernel and
eventually calls into a memory allocator, this conflicts with the RT
semantics.

Convert it to a local_lock which allows RT kernels to substitute them with
a real per CPU lock. On non RT kernels this maps to preempt_disable() as
before, but provides also lockdep coverage of the critical region.
No functional change.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Acked-by: Peter Zijlstra <peterz@infradead.org>
Link: https://lore.kernel.org/r/20200527201119.1692513-3-bigeasy@linutronix.de