Re: [PATCH] powerpc/xive: Do not skip CPU-less nodes when creating the IPIs

On PowerVM, CPU-less nodes can be populated with hot-plugged CPUs at
runtime. Today, the IPI is not created for such nodes, and hot-plugged
CPUs use a bogus IPI, which leads to soft lockups.

We could create the node IPI on demand but it is a bit complex because
this code would be called under bringup_up() and some IRQ locking is
being done. The simplest solution is to create the IPIs for all nodes
at startup.

Fixes: 7dcc37b3eff9 ("powerpc/xive: Map one IPI interrupt per node")
Cc: stable@vger.kernel.org # v5.13
Reported-by: Geetika Moolchandani <redacted>
Cc: Srikar Dronamraju <redacted>
Signed-off-by: Cédric Le Goater <clg@kaod.org>
---

This patch breaks old versions of irqbalance (<= v1.4). Possible nodes
are collected from /sys/devices/system/node/ but CPU-less nodes are
not listed there. When interrupts are scanned, the link representing
the node structure is NULL and segfault occurs.

Version 1.7 seems immune. 

---
 arch/powerpc/sysdev/xive/common.c | 4 ----
 1 file changed, 4 deletions(-)

diff --git a/arch/powerpc/sysdev/xive/common.c b/arch/powerpc/sysdev/xive/common.c
index f3b16ed48b05..5d2c58dba57e 100644
--- a/arch/powerpc/sysdev/xive/common.c
+++ b/arch/powerpc/sysdev/xive/common.c

@@ -1143,10 +1143,6 @@ static int __init xive_request_ipi(void)
 		struct xive_ipi_desc *xid = &xive_ipis[node];
 		struct xive_ipi_alloc_info info = { node };
 
-		/* Skip nodes without CPUs */
-		if (cpumask_empty(cpumask_of_node(node)))
-			continue;
-
 		/*
 		 * Map one IPI interrupt per node for all cpus of that node.
 		 * Since the HW interrupt number doesn't have any meaning,