From: Sebastian Andrzej Siewior Date: Mon, 9 Nov 2020 21:30:41 +0100 Subject: [PATCH 1/2] kthread: Move prio/affinite change into the newly created thread With enabled threaded interrupts the nouveau driver reported the following: | Chain exists of: | &mm->mmap_lock#2 --> &device->mutex --> &cpuset_rwsem | | Possible unsafe locking scenario: | | CPU0 CPU1 | ---- ---- | lock(&cpuset_rwsem); | lock(&device->mutex); | lock(&cpuset_rwsem); | lock(&mm->mmap_lock#2); The device->mutex is nvkm_device::mutex. Unblocking the lockchain at `cpuset_rwsem' is probably the easiest thing to do. Move the priority reset to the start of the newly created thread. Fixes: 710da3c8ea7df ("sched/core: Prevent race condition between cpuset and __sched_setscheduler()") Reported-by: Mike Galbraith Signed-off-by: Sebastian Andrzej Siewior Link: https://lkml.kernel.org/r/a23a826af7c108ea5651e73b8fbae5e653f16e86.camel@gmx.de --- kernel/kthread.c | 16 ++++++++-------- 1 file changed, 8 insertions(+), 8 deletions(-) --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -243,6 +243,7 @@ EXPORT_SYMBOL_GPL(kthread_parkme); static int kthread(void *_create) { + static const struct sched_param param = { .sched_priority = 0 }; /* Copy data: it's on kthread's stack */ struct kthread_create_info *create = _create; int (*threadfn)(void *data) = create->threadfn; @@ -273,6 +274,13 @@ static int kthread(void *_create) init_completion(&self->parked); current->vfork_done = &self->exited; + /* + * The new thread inherited kthreadd's priority and CPU mask. Reset + * back to default in case they have been changed. + */ + sched_setscheduler_nocheck(current, SCHED_NORMAL, ¶m); + set_cpus_allowed_ptr(current, housekeeping_cpumask(HK_FLAG_KTHREAD)); + /* OK, tell user we're spawned, wait for stop or wakeup */ __set_current_state(TASK_UNINTERRUPTIBLE); create->result = current; @@ -370,7 +378,6 @@ struct task_struct *__kthread_create_on_ } task = create->result; if (!IS_ERR(task)) { - static const struct sched_param param = { .sched_priority = 0 }; char name[TASK_COMM_LEN]; /* @@ -379,13 +386,6 @@ struct task_struct *__kthread_create_on_ */ vsnprintf(name, sizeof(name), namefmt, args); set_task_comm(task, name); - /* - * root may have changed our (kthreadd's) priority or CPU mask. - * The kernel thread should not inherit these properties. - */ - sched_setscheduler_nocheck(task, SCHED_NORMAL, ¶m); - set_cpus_allowed_ptr(task, - housekeeping_cpumask(HK_FLAG_KTHREAD)); } kfree(create); return task;