linux-kernel - [PATCH v7] workqueue: Fix edge cases for calc of pool's cpumask

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Message-Id: <1f06a3f5-ddb6-ee3f-3496-12e081452232@linux.vnet.ibm.com>
Date:   Thu, 27 Jul 2017 16:27:14 -0500
From:   Michael Bringmann <mwb@...ux.vnet.ibm.com>
To:     Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     nfont@...ux.vnet.ibm.com,
        Michael Bringmann <mwb@...ux.vnet.ibm.com>
Subject: [PATCH v7] workqueue: Fix 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 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@...nel.org>]

Signed-off-by: Michael Bringmann <mwb@...ux.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: