Re: [PATCH] workqueue: fix selection of wake_cpu in kick_pool()

[Tejun Heo <tj@kernel.org>]

Hello,

On Mon, Apr 15, 2024 at 07:35:49AM +0200, Sven Schnelle wrote:
> With cpu_possible_mask=0-63 and cpu_online_mask=0-7 the following
> kernel oops was observed:
> 
> smp: Bringing up secondary CPUs ...
> smp: Brought up 1 node, 8 CPUs
> Unable to handle kernel pointer dereference in virtual kernel address space
> Failing address: 0000000000000000 TEID: 0000000000000803
> [..]
>  Call Trace:
> arch_vcpu_is_preempted+0x12/0x80
> select_idle_sibling+0x42/0x560
> select_task_rq_fair+0x29a/0x3b0
> try_to_wake_up+0x38e/0x6e0
> kick_pool+0xa4/0x198
> __queue_work.part.0+0x2bc/0x3a8
> call_timer_fn+0x36/0x160
> __run_timers+0x1e2/0x328
> __run_timer_base+0x5a/0x88
> run_timer_softirq+0x40/0x78
> __do_softirq+0x118/0x388
> irq_exit_rcu+0xc0/0xd8
> do_ext_irq+0xae/0x168
> ext_int_handler+0xbe/0xf0
> psw_idle_exit+0x0/0xc
> default_idle_call+0x3c/0x110
> do_idle+0xd4/0x158
> cpu_startup_entry+0x40/0x48
> rest_init+0xc6/0xc8
> start_kernel+0x3c4/0x5e0
> startup_continue+0x3c/0x50
> 
> The crash is caused by calling arch_vcpu_is_preempted() for an offline
> CPU. To avoid this, select the cpu with cpumask_any_and_distribute()
> to mask __pod_cpumask with cpu_online_mask.
> 
> Fixes: 8639ecebc9b1 ("workqueue: Implement non-strict affinity scope for unbound workqueues")
> Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
> ---
>  kernel/workqueue.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/workqueue.c b/kernel/workqueue.c
> index 0066c8f6c154..d02b0c02c9e2 100644
> --- a/kernel/workqueue.c
> +++ b/kernel/workqueue.c
> @@ -1277,7 +1277,8 @@ static bool kick_pool(struct worker_pool *pool)
>  	    !cpumask_test_cpu(p->wake_cpu, pool->attrs->__pod_cpumask)) {
>  		struct work_struct *work = list_first_entry(&pool->worklist,
>  						struct work_struct, entry);
> -		p->wake_cpu = cpumask_any_distribute(pool->attrs->__pod_cpumask);
> +		p->wake_cpu = cpumask_any_and_distribute(pool->attrs->__pod_cpumask,
> +							 cpu_online_mask);

So, this wouldn't necessarily fix the problem completely but regardless of
how that's plugged, this is still something we want to do to avoid picking
offline CPUs. Can you please update the patch description accordingly and
resend?

Thanks.

-- 
tejun