[PATCH 2/3] workqueue: use RCU accessors when populating wq->cpu_pwq

[Breno Leitao <leitao@debian.org>]

wq->cpu_pwq holds RCU-protected pwq pointers, but the percpu allocation
path fills it in with plain loads and stores, which sparse flags:

  kernel/workqueue.c:5682:57: sparse: incorrect type in initializer (different address spaces) @@     expected struct pool_workqueue **pwq_p @@     got struct pool_workqueue [noderef] __rcu ** @@

Allocate the array as __rcu pointers and publish each pwq with
rcu_assign_pointer() once it is initialized and linked, the order
install_unbound_pwq() uses.

The warnings are not new: commit 79f23600bc7b ("workqueue: factor out
get_percpu_pool()") only turned the flagged assignment into an
initializer.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202608120931.tvTzq1gD-lkp@intel.com/
Signed-off-by: Breno Leitao <leitao@debian.org>
---
 kernel/workqueue.c | 19 ++++++++++++-------
 1 file changed, 12 insertions(+), 7 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 1748ef5541a26..83aced28523b5 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -5681,21 +5681,23 @@ static void unbound_wq_update_pwq(struct workqueue_struct *wq, int cpu)
 
 static int alloc_and_link_percpu_pwqs(struct workqueue_struct *wq)
 {
+	struct pool_workqueue *pwq;
 	int cpu;
 
 	for_each_possible_cpu(cpu) {
-		struct pool_workqueue **pwq_p = per_cpu_ptr(wq->cpu_pwq, cpu);
 		struct worker_pool *pool = get_percpu_pool(wq, cpu);
 
-		*pwq_p = kmem_cache_alloc_node(pwq_cache, GFP_KERNEL, pool->node);
-		if (!*pwq_p)
+		pwq = kmem_cache_alloc_node(pwq_cache, GFP_KERNEL, pool->node);
+		if (!pwq)
 			return -ENOMEM;
 
-		init_pwq(*pwq_p, wq, pool);
+		init_pwq(pwq, wq, pool);
 
 		mutex_lock(&wq->mutex);
-		link_pwq(*pwq_p);
+		link_pwq(pwq);
 		mutex_unlock(&wq->mutex);
+
+		rcu_assign_pointer(*per_cpu_ptr(wq->cpu_pwq, cpu), pwq);
 	}
 
 	return 0;
@@ -5708,7 +5710,7 @@ static int alloc_and_link_pwqs(struct workqueue_struct *wq)
 
 	lockdep_assert_held(&wq_pool_mutex);
 
-	wq->cpu_pwq = alloc_percpu(struct pool_workqueue *);
+	wq->cpu_pwq = alloc_percpu(struct pool_workqueue __rcu *);
 	if (!wq->cpu_pwq)
 		goto enomem;
 
@@ -5734,8 +5736,11 @@ static int alloc_and_link_pwqs(struct workqueue_struct *wq)
 enomem:
 	if (wq->cpu_pwq) {
 		for_each_possible_cpu(cpu) {
-			struct pool_workqueue *pwq = *per_cpu_ptr(wq->cpu_pwq, cpu);
+			struct pool_workqueue __rcu **slot;
+			struct pool_workqueue *pwq;
 
+			slot = per_cpu_ptr(wq->cpu_pwq, cpu);
+			pwq = rcu_access_pointer(*slot);
 			if (pwq) {
 				/*
 				 * Unlink pwq from wq->pwqs since link_pwq()

-- 
2.53.0-Meta