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

[Sven Schnelle <svens@linux.ibm.com>]

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);
 		get_work_pwq(work)->stats[PWQ_STAT_REPATRIATED]++;
 	}
 #endif
-- 
2.40.1