[PATCH] x86/resctrl: Fix mbm_setup_overflow_handler() when last CPU goes offline

[Tony Luck]

Don't bother looking for another CPU to take over MBM overflow duties
when the last CPU in a domain goes offline. Doing so results in this
Oops:

[   97.166136] BUG: kernel NULL pointer dereference, address: 0000000000000000
[   97.173118] #PF: supervisor read access in kernel mode
[   97.178263] #PF: error_code(0x0000) - not-present page
[   97.183410] PGD 0
[   97.185438] Oops: 0000 [#1] PREEMPT SMP NOPTI
[   97.189805] CPU: 36 PID: 235 Comm: cpuhp/36 Tainted: G                T  6.9.0-rc1 #356
[   97.208322] RIP: 0010:__find_nth_andnot_bit+0x66/0x110

Fixes: 978fcca954cb ("x86/resctrl: Allow overflow/limbo handlers to be scheduled on any-but CPU")
Signed-off-by: Tony Luck <tony.luck@xxxxxxxxx>
---
 arch/x86/kernel/cpu/resctrl/monitor.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/arch/x86/kernel/cpu/resctrl/monitor.c b/arch/x86/kernel/cpu/resctrl/monitor.c
index 757d475158a3..4d9987acffd6 100644
--- a/arch/x86/kernel/cpu/resctrl/monitor.c
+++ b/arch/x86/kernel/cpu/resctrl/monitor.c
@@ -929,6 +929,10 @@ void mbm_setup_overflow_handler(struct rdt_domain *dom, unsigned long delay_ms,
 	unsigned long delay = msecs_to_jiffies(delay_ms);
 	int cpu;
 
+	/* Nothing to do if this is the last CPU in a domain going offline */
+	if (!delay_ms && bitmap_weight(cpumask_bits(&dom->cpu_mask), nr_cpu_ids) == 1)
+		return;
+
 	/*
 	 * When a domain comes online there is no guarantee the filesystem is
 	 * mounted. If not, there is no need to catch counter overflow.
-- 
2.44.0