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Subject[tip: x86/urgent] genirq/matrix: Deal with the sillyness of for_each_cpu() on UP
The following commit has been merged into the x86/urgent branch of tip:

Commit-ID: 784a0830377d0761834e385975bc46861fea9fa0
Author: Thomas Gleixner <>
AuthorDate: Sun, 30 Aug 2020 19:07:53 +02:00
Committer: Thomas Gleixner <>
CommitterDate: Sun, 30 Aug 2020 19:17:28 +02:00

genirq/matrix: Deal with the sillyness of for_each_cpu() on UP

Most of the CPU mask operations behave the same way, but for_each_cpu() and
it's variants ignore the cpumask argument and claim that CPU0 is always in
the mask. This is historical, inconsistent and annoying behaviour.

The matrix allocator uses for_each_cpu() and can be called on UP with an
empty cpumask. The calling code does not expect that this succeeds but
until commit e027fffff799 ("x86/irq: Unbreak interrupt affinity setting")
this went unnoticed. That commit added a WARN_ON() to catch cases which
move an interrupt from one vector to another on the same CPU. The warning
triggers on UP.

Add a check for the cpumask being empty to prevent this.

Fixes: 2f75d9e1c905 ("genirq: Implement bitmap matrix allocator")
Reported-by: kernel test robot <>
Signed-off-by: Thomas Gleixner <>
kernel/irq/matrix.c | 7 +++++++
1 file changed, 7 insertions(+)

diff --git a/kernel/irq/matrix.c b/kernel/irq/matrix.c
index 30cc217..651a4ad 100644
--- a/kernel/irq/matrix.c
+++ b/kernel/irq/matrix.c
@@ -380,6 +380,13 @@ int irq_matrix_alloc(struct irq_matrix *m, const struct cpumask *msk,
unsigned int cpu, bit;
struct cpumap *cm;

+ /*
+ * Not required in theory, but matrix_find_best_cpu() uses
+ * for_each_cpu() which ignores the cpumask on UP .
+ */
+ if (cpumask_empty(msk))
+ return -EINVAL;
cpu = matrix_find_best_cpu(m, msk);
if (cpu == UINT_MAX)
return -ENOSPC;
 \ /
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