module: replace use of system_wq with system_dfl_wq

Currently if a user enqueues a work item using schedule_delayed_work() the
used wq is "system_wq" (per-cpu wq) while queue_delayed_work() use
WORK_CPU_UNBOUND (used when a cpu is not specified). The same applies to
schedule_work() that is using system_wq and queue_work(), that makes use
again of WORK_CPU_UNBOUND.

This lack of consistency cannot be addressed without refactoring the API.

This continues the effort to refactor workqueue APIs, which began with
the introduction of new workqueues and a new alloc_workqueue flag in:

commit 128ea9f ("workqueue: Add system_percpu_wq and system_dfl_wq")
commit 930c2ea ("workqueue: Add new WQ_PERCPU flag")

Switch to using system_dfl_wq, the new unbound workqueue, because the
users do not benefit from a per-cpu workqueue.

Suggested-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Marco Crivellari <marco.crivellari@suse.com>
Reviewed-by: Petr Pavlu <petr.pavlu@suse.com>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>

Author: DispatchCode

kernel/module/dups.c

@@ -113,7 +113,7 @@ static void kmod_dup_request_complete(struct work_struct *work)
 	 * let this linger forever as this is just a boot optimization for
 	 * possible abuses of vmalloc() incurred by finit_module() thrashing.
 	 */
-	queue_delayed_work(system_wq, &kmod_req->delete_work, 60 * HZ);
+	queue_delayed_work(system_dfl_wq, &kmod_req->delete_work, 60 * HZ);
 }
 
 bool kmod_dup_request_exists_wait(char *module_name, bool wait, int *dup_ret)
@@ -240,7 +240,7 @@ void kmod_dup_request_announce(char *module_name, int ret)
 	 * There is no rush. But we also don't want to hold the
 	 * caller up forever or introduce any boot delays.
 	 */
-	queue_work(system_wq, &kmod_req->complete_work);
+	queue_work(system_dfl_wq, &kmod_req->complete_work);
 
 out:
 	mutex_unlock(&kmod_dup_mutex);