[RFC PATCH 1/8] workqueue: Unconditionally set cpumask in worker_attach_to_pool()

[Lai Jiangshan]

From: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx>

If a worker is spuriously woken up after kthread_bind_mask() but before
worker_attach_to_pool(), and there are some cpu-hot-[un]plug happening
during the same interval, the worker task might be pushed away from its
bound CPU with its affinity changed by the scheduler and worker_attach_to_pool()
doesn't rebind it properly.

Do unconditionally affinity binding in worker_attach_to_pool() to fix
the problem.

Prepare for moving worker_attach_to_pool() from create_worker() to the
starting of worker_thread() which will really cause the said interval
even without spurious wakeup.

Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Tejun Heo <tj@xxxxxxxxxx>,
Cc: Petr Mladek <pmladek@xxxxxxxx>
Cc: Michal Hocko <mhocko@xxxxxxxx>,
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>,
Cc: Wedson Almeida Filho <wedsonaf@xxxxxxxxxx>
Fixes: 640f17c82460 ("workqueue: Restrict affinity change to rescuer")
Signed-off-by: Lai Jiangshan <jiangshan.ljs@xxxxxxxxxxxx>
---
 kernel/workqueue.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 1ea50f6be843..928aad7d6123 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1872,8 +1872,11 @@ static void worker_attach_to_pool(struct worker *worker,
 	else
 		kthread_set_per_cpu(worker->task, pool->cpu);
 
-	if (worker->rescue_wq)
-		set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
+	/*
+	 * set_cpus_allowed_ptr() will fail if the cpumask doesn't have any
+	 * online CPUs.  It'll be re-applied when any of the CPUs come up.
+	 */
+	set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
 
 	list_add_tail(&worker->node, &pool->workers);
 	worker->pool = pool;
-- 
2.19.1.6.gb485710b