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>

6 files changed, 19 insertions, 24 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index d35d2b6ddbfb..e5c64f7b8c1d 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1793,7 +1793,7 @@ extern void thread_group_cputime_adjusted(struct task_struct *p, cputime_t *ut,
 #define PF_SWAPWRITE    0x00800000      /* Allowed to write to swap */
 #define PF_SPREAD_PAGE  0x01000000      /* Spread page cache over cpuset */
 #define PF_SPREAD_SLAB  0x02000000      /* Spread some slab caches over cpuset */
-#define PF_THREAD_BOUND 0x04000000      /* Thread bound to specific cpu */
+#define PF_NO_SETAFFINITY 0x04000000    /* Userland is not allowed to meddle with cpus_allowed */
 #define PF_MCE_EARLY    0x08000000      /* Early kill for mce process policy */
 #define PF_MEMPOLICY    0x10000000      /* Non-default NUMA mempolicy */
 #define PF_MUTEX_TESTER 0x20000000      /* Thread belongs to the rt mutex tester */
diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index a32f9432666c..3852d926322c 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -2224,11 +2224,11 @@ retry_find_task:
                 tsk = tsk->group_leader;
 
         /*
-         * Workqueue threads may acquire PF_THREAD_BOUND and become
+         * Workqueue threads may acquire PF_NO_SETAFFINITY and become
          * trapped in a cpuset, or RT worker may be born in a cgroup
          * with no rt_runtime allocated.  Just say no.
          */
-        if (tsk == kthreadd_task || (tsk->flags & PF_THREAD_BOUND)) {
+        if (tsk == kthreadd_task || (tsk->flags & PF_NO_SETAFFINITY)) {
                 ret = -EINVAL;
                 rcu_read_unlock();
                 goto out_unlock_cgroup;
diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 4f9dfe43ecbd..f22e94792707 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1388,16 +1388,16 @@ static int cpuset_can_attach(struct cgroup *cgrp, struct cgroup_taskset *tset)
 
         cgroup_taskset_for_each(task, cgrp, tset) {
                 /*
-                 * Kthreads bound to specific cpus cannot be moved to a new
-                 * cpuset; we cannot change their cpu affinity and
-                 * isolating such threads by their set of allowed nodes is
-                 * unnecessary.  Thus, cpusets are not applicable for such
-                 * threads.  This prevents checking for success of
-                 * set_cpus_allowed_ptr() on all attached tasks before
-                 * cpus_allowed may be changed.
+                 * Kthreads which disallow setaffinity shouldn't be moved
+                 * to a new cpuset; we don't want to change their cpu
+                 * affinity and isolating such threads by their set of
+                 * allowed nodes is unnecessary.  Thus, cpusets are not
+                 * applicable for such threads.  This prevents checking for
+                 * success of set_cpus_allowed_ptr() on all attached tasks
+                 * before cpus_allowed may be changed.
                  */
                 ret = -EINVAL;
-                if (task->flags & PF_THREAD_BOUND)
+                if (task->flags & PF_NO_SETAFFINITY)
                         goto out_unlock;
                 ret = security_task_setscheduler(task);
                 if (ret)
diff --git a/kernel/kthread.c b/kernel/kthread.c
index 691dc2ef9baf..a2fbbb782bad 100644
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -260,7 +260,7 @@ static void __kthread_bind(struct task_struct *p, unsigned int cpu)
 {
         /* It's safe because the task is inactive. */
         do_set_cpus_allowed(p, cpumask_of(cpu));
-        p->flags |= PF_THREAD_BOUND;
+        p->flags |= PF_NO_SETAFFINITY;
 }
 
 /**
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7f12624a393c..23606ee961b5 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4126,6 +4126,10 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
         get_task_struct(p);
         rcu_read_unlock();
 
+        if (p->flags & PF_NO_SETAFFINITY) {
+                retval = -EINVAL;
+                goto out_put_task;
+        }
         if (!alloc_cpumask_var(&cpus_allowed, GFP_KERNEL)) {
                 retval = -ENOMEM;
                 goto out_put_task;
@@ -4773,11 +4777,6 @@ int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
                 goto out;
         }
 
-        if (unlikely((p->flags & PF_THREAD_BOUND) && p != current)) {
-                ret = -EINVAL;
-                goto out;
-        }
-
         do_set_cpus_allowed(p, new_mask);
 
         /* Can the task run on the task's current CPU? If so, we're done */
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);
         }