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Subject[PATCH v7] workqueue: Fix edge cases for calc of pool's cpumask
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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 an
intermediate fix 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>
---
Changes in V7:
-- Update descriptive text
---
kernel/workqueue.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index c74bf39..ae3e5ac 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: online intersect > "
+ "possible intersect\n");
+ return false;
+ }
+
return !cpumask_equal(cpumask, attrs->cpumask);

use_dfl:
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