[PATCH v2] riscv: smp: set CPU 0 possible in setup_smp()

[Paul Sherman]

setup_smp() calls set_cpu_possible() for CPUs 1..nr_cpu_ids-1 but
never for CPU 0 (the boot CPU). x86 handles this via
init_cpu_possible(cpumask_of(0)); RISC-V has no equivalent.

Without CPU 0 in cpu_possible_mask, rcu_init_one()'s
for_each_possible_cpu() loop skips it, leaving rdp->mynode=NULL.
rcutree_prepare_cpu() then dereferences NULL and hangs.

Exposed on Sophgo SG2042 (64-hart, 4-NUMA) with Linux 7.2-rc3.

Fixes: a4166aec1130 ("riscv: Deduplicate code in setup_smp()")
Signed-off-by: Paul Sherman <shermanpauldylan at gmail.com>
---
 arch/riscv/kernel/smpboot.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/riscv/kernel/smpboot.c b/arch/riscv/kernel/smpboot.c
index f6ef57930b50..8d3a437c0f30 100644
--- a/arch/riscv/kernel/smpboot.c
+++ b/arch/riscv/kernel/smpboot.c
@@ -171,6 +171,15 @@ void __init setup_smp(void)
 	for (cpuid = 1; cpuid < nr_cpu_ids; cpuid++)
 		if (cpuid_to_hartid_map(cpuid) != INVALID_HARTID)
 			set_cpu_possible(cpuid, true);
+
+	/*
+	 * Boot CPU (cpuid=0) is never set possible by the loop above.
+	 * x86 and arm64 call init_cpu_possible(cpumask_of(0)) explicitly;
+	 * RISC-V was missing this. Without it, rcu_init_one()'s
+	 * for_each_possible_cpu loop skips CPU 0, leaving rdp->mynode=NULL,
+	 * causing rcutree_prepare_cpu() to dereference NULL and hang.
+	 */
+	set_cpu_possible(0, true);
 }
 
 static int start_secondary_cpu(int cpu, struct task_struct *tidle)
-- 
2.53.0