sched: replace PF_THREAD_BOUND with PF_NO_SETAFFINITY

PF_THREAD_BOUND was originally used to mark kernel threads which were
bound to a specific CPU using kthread_bind() and a task with the flag
set allows cpus_allowed modifications only to itself.  Workqueue is
currently abusing it to prevent userland from meddling with
cpus_allowed of workqueue workers.

What we need is a flag to prevent userland from messing with
cpus_allowed of certain kernel tasks.  In kernel, anyone can
(incorrectly) squash the flag, and, for worker-type usages,
restricting cpus_allowed modification to the task itself doesn't
provide meaningful extra proection as other tasks can inject work
items to the task anyway.

This patch replaces PF_THREAD_BOUND with PF_NO_SETAFFINITY.
sched_setaffinity() checks the flag and return -EINVAL if set.
set_cpus_allowed_ptr() is no longer affected by the flag.

This will allow simplifying workqueue worker CPU affinity management.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 3 insertions, 7 deletions

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 969be0b72071..39a591f65b08 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1757,12 +1757,8 @@ static struct worker *create_worker(struct worker_pool *pool)
         set_user_nice(worker->task, pool->attrs->nice);
         set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
 
-        /*
-         * %PF_THREAD_BOUND is used to prevent userland from meddling with
-         * cpumask of workqueue workers.  This is an abuse.  We need
-         * %PF_NO_SETAFFINITY.
-         */
-        worker->task->flags |= PF_THREAD_BOUND;
+        /* prevent userland from meddling with cpumask of workqueue workers */
+        worker->task->flags |= PF_NO_SETAFFINITY;
 
         /*
          * The caller is responsible for ensuring %POOL_DISASSOCIATED
@@ -3876,7 +3872,7 @@ struct workqueue_struct *__alloc_workqueue_key(const char *fmt,
                 }
 
                 wq->rescuer = rescuer;
-                rescuer->task->flags |= PF_THREAD_BOUND;
+                rescuer->task->flags |= PF_NO_SETAFFINITY;
                 wake_up_process(rescuer->task);
         }