Re: [PATCH] cpu: Fix online housekeeping CPU check in _cpu_down()

[Bradley Morgan]

On 8 November 2026 18:08:49 GMT, Waiman Long <longman@xxxxxxxxxx> wrote:
>There is currently a check in _cpu_down() to make sure that there is
>at least one online housekeeping (HK) CPU left after the shutdown of
>the given @cpu parameter. However the cpu_online_mask used for the
>check may include the current @cpu to be shut down. So the fact that
>there is at least one online housekeeping CPU at the moment does not
>guarantee that there will be any online housekeeping CPU left for the
>sched domain after the shutdown.
>
>Fix that by making sure that there are at least 2 online housekeeping
>CPUs available if @cpu is currently a housekeeping CPU.
>
>Fixes: de715325cc47 ("cpu: Revert "cpu/hotplug: Prevent self deadlock on CPU hot-unplug"")
>Signed-off-by: Waiman Long <longman@xxxxxxxxxx>
>---
> kernel/cpu.c | 11 +++++++----
> 1 file changed, 7 insertions(+), 4 deletions(-)
>
>diff --git a/kernel/cpu.c b/kernel/cpu.c
>index b3c8553d7bd6..ed641ee6b37c 100644
>--- a/kernel/cpu.c
>+++ b/kernel/cpu.c
>@@ -1415,11 +1415,14 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen,
> 	cpus_write_lock();
> 
> 	/*
>-	 * Keep at least one housekeeping cpu onlined to avoid generating
>-	 * an empty sched_domain span.
>+	 * Keep at least one housekeeping cpu online to avoid generating an
>+	 * empty sched_domain span. As cpu_online_mask includes the @cpu to be
>+	 * shut down, there should be at least 2 online HK CPUs at the moment
>+	 * if @cpu is currently a HK CPU.
> 	 */
>-	if (cpumask_any_and(cpu_online_mask,
>-			    housekeeping_cpumask(HK_TYPE_DOMAIN)) >= nr_cpu_ids) {
>+	if (cpumask_test_cpu(cpu, housekeeping_cpumask(HK_TYPE_DOMAIN)) &&
>+	   (cpumask_weight_and(cpu_online_mask,
>+			       housekeeping_cpumask(HK_TYPE_DOMAIN)) < 2)) {

Well, this patch is good, reasonable enough to me, I may be odd sometimes,
but this patch is sane enough.

Reviewed-by: Bradley Morgan <brads@xxxxxxxxxxxxxx>


> 		ret = -EBUSY;
> 		goto out;
> 	}
>

--- Thanks!