[PATCH 5.8 206/255] genirq/matrix: Deal with the sillyness of for_each_cpu() on UP

[Greg Kroah-Hartman]

From: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

commit 784a0830377d0761834e385975bc46861fea9fa0 upstream.

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 <rong.a.chen@xxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: stable@xxxxxxxxxxxxxxx
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
 kernel/irq/matrix.c |    7 +++++++
 1 file changed, 7 insertions(+)

--- a/kernel/irq/matrix.c
+++ b/kernel/irq/matrix.c
@@ -380,6 +380,13 @@ int irq_matrix_alloc(struct irq_matrix *
 	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;