rcu: Make Tiny RCU explicitly disable preemption

Because Tiny RCU is used only in kernels built with either
CONFIG_PREEMPT_NONE=y or CONFIG_PREEMPT_VOLUNTARY=y, there has not been
any need for TINY RCU to explicitly disable preemption.  However, the
prospect of lazy preemption changes that, and preemption means that
the non-atomic increment in synchronize_rcu() can be preempted, with
the possibility that one of the increments is lost.  This could cause
failures for users of the APIs that poll RCU grace periods.

This commit therefore adds the needed preempt_disable() and
preempt_enable() call to Tiny RCU.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Ankur Arora <ankur.a.arora@oracle.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

kernel/rcu/tiny.c

@@ -153,7 +153,9 @@ void synchronize_rcu(void)
 			 lock_is_held(&rcu_lock_map) ||
 			 lock_is_held(&rcu_sched_lock_map),
 			 "Illegal synchronize_rcu() in RCU read-side critical section");
+	preempt_disable();
 	WRITE_ONCE(rcu_ctrlblk.gp_seq, rcu_ctrlblk.gp_seq + 2);
+	preempt_enable();
 }
 EXPORT_SYMBOL_GPL(synchronize_rcu);