srcu: Take early exit on memory-allocation failure

It turns out that init_srcu_struct() can be invoked from usermode tasks,
and that fatal signals received by these tasks can cause memory-allocation
failures.  These failures are not handled well by init_srcu_struct(),
so much so that NULL pointer dereferences can result.  This commit
therefore causes init_srcu_struct() to take an early exit upon detection
of memory-allocation failure.

Link: https://lore.kernel.org/lkml/20200908144306.33355-1-aik@ozlabs.ru/
Reported-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Tested-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

kernel/rcu/srcutree.c

@@ -177,11 +177,13 @@ static int init_srcu_struct_fields(struct srcu_struct *ssp, bool is_static)
 	INIT_DELAYED_WORK(&ssp->work, process_srcu);
 	if (!is_static)
 		ssp->sda = alloc_percpu(struct srcu_data);
+	if (!ssp->sda)
+		return -ENOMEM;
 	init_srcu_struct_nodes(ssp, is_static);
 	ssp->srcu_gp_seq_needed_exp = 0;
 	ssp->srcu_last_gp_end = ktime_get_mono_fast_ns();
 	smp_store_release(&ssp->srcu_gp_seq_needed, 0); /* Init done. */
-	return ssp->sda ? 0 : -ENOMEM;
+	return 0;
 }
 
 #ifdef CONFIG_DEBUG_LOCK_ALLOC