[PATCH v5] workqueue: Fix edge cases for calc of pool's cpumask

[Michael Bringmann <mwb@linux.vnet.ibm.com>]

On NUMA systems with dynamic processors, the content of the cpumask
may change over time.  As new processors are added via DLPAR operations,
workqueues are created for them.  Depending upon the order in which CPUs
are added/removed, we may run into problems with the content of the
cpumask used by the workqueues.  This patch deals with situations where
the online cpumask for a node is a proper superset of possible cpumask
for the node.  It also deals with edge cases where the order in which
CPUs are removed/added from the online cpumask may leave the set for a
node empty, and require execution by CPUs on another node.

In these and other cases, the patch attempts to ensure that a valid,
usable cpumask is used to set up newly created pools for workqueues.

[With additions to the patch provided by Tejun Hao <tj@kernel.org>]

Signed-off-by: Michael Bringmann <mwb@linux.vnet.ibm.com>
---
Changes in V5:
  -- Revise the warning notification.
---
 kernel/workqueue.c |    7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index c74bf39..6b6d540 100644
--- a/kernel/workqueue.c
+++ b/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: onl intersect > "
+				"possible intersect\n");
+		return false;
+	}
+
 	return !cpumask_equal(cpumask, attrs->cpumask);
 
 use_dfl: