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Subject[PATCH REPOST v3.9-rc1] 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>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
---
Reposted because the original posting forgot to cc lkml.

Thanks.

include/linux/sched.h | 2 +-
kernel/cgroup.c | 4 ++--
kernel/cpuset.c | 16 ++++++++--------
kernel/kthread.c | 2 +-
kernel/sched/core.c | 9 ++++-----
kernel/workqueue.c | 6 +++---
6 files changed, 19 insertions(+), 20 deletions(-)

--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -1793,7 +1793,7 @@ extern void thread_group_cputime_adjuste
#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 */
--- 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;
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -1388,16 +1388,16 @@ static int cpuset_can_attach(struct cgro

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)
--- a/kernel/kthread.c
+++ b/kernel/kthread.c
@@ -260,7 +260,7 @@ static void __kthread_bind(struct task_s
{
/* 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;
}

/**
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4126,6 +4126,10 @@ long sched_setaffinity(pid_t pid, const
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;
@@ -4772,11 +4776,6 @@ int set_cpus_allowed_ptr(struct task_str
ret = -EINVAL;
goto out;
}
-
- if (unlikely((p->flags & PF_THREAD_BOUND) && p != current)) {
- ret = -EINVAL;
- goto out;
- }

do_set_cpus_allowed(p, new_mask);

--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1741,12 +1741,12 @@ static struct worker *create_worker(stru
* above the flag definition for details.
*
* As an unbound worker may later become a regular one if CPU comes
- * online, make sure every worker has %PF_THREAD_BOUND set.
+ * online, make sure every worker has %PF_NO_SETAFFINITY set.
*/
if (!(pool->flags & POOL_DISASSOCIATED)) {
kthread_bind(worker->task, pool->cpu);
} else {
- worker->task->flags |= PF_THREAD_BOUND;
+ worker->task->flags |= PF_NO_SETAFFINITY;
worker->flags |= WORKER_UNBOUND;
}

@@ -3219,7 +3219,7 @@ struct workqueue_struct *__alloc_workque
if (IS_ERR(rescuer->task))
goto err;

- rescuer->task->flags |= PF_THREAD_BOUND;
+ rescuer->task->flags |= PF_NO_SETAFFINITY;
wake_up_process(rescuer->task);
}


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