workqueue: Work around edge cases for calc of pool's cpumask

There is an underlying assumption/trade-off in many layers of the Linux
system that CPU <-> node mapping is static.  This is despite the presence
of features like NUMA and 'hotplug' that support the dynamic addition/
removal of fundamental system resources like CPUs and memory.  PowerPC
systems, however, do provide extensive features for the dynamic change
of resources available to a system.

Currently, there is little or no synchronization protection around the
updating of the CPU <-> node mapping, and the export/update of this
information for other layers / modules.  In systems which can change
this mapping during 'hotplug', like PowerPC, the information is changing
underneath all layers that might reference it.

This patch attempts to ensure that a valid, usable cpumask attribute
is used by the workqueue infrastructure when setting up new resource
pools.  It prevents a crash that has been observed when an 'empty'
cpumask is passed along to the worker/task scheduling code.  It is
intended as a temporary workaround until a more fundamental review and
correction of the issue can be done.

[With additions to the patch provided by Tejun Hao <tj@kernel.org>]
Signed-off-by: Michael Bringmann <mwb@linux.vnet.ibm.com>
Signed-off-by: Tejun Heo <tj@kernel.org>

kernel/workqueue.c

@@ -3577,6 +3577,13 @@ static bool wq_calc_node_cpumask(const struct workqueue_attrs *attrs, int node,
 
 	/* yeap, return possible CPUs in @node that @attrs wants */
 	cpumask_and(cpumask, attrs->cpumask, wq_numa_possible_cpumask[node]);
+
+	if (cpumask_empty(cpumask)) {
+		pr_warn_once("WARNING: workqueue cpumask: online intersect > "
+				"possible intersect\n");
+		return false;
+	}
+
 	return !cpumask_equal(cpumask, attrs->cpumask);
 
 use_dfl: