From cf380a4a96e2260742051fa7fc831596bb26cc8b Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Tue, 29 Nov 2016 18:51:07 +0100 Subject: [PATCH] kthread: Don't use to_live_kthread() in kthread_[un]park() Now that to_kthread() is always validm change kthread_park() and kthread_unpark() to use it and kill to_live_kthread(). The conversion of kthread_unpark() is trivial. If KTHREAD_IS_PARKED is set then the task has called complete(&self->parked) and there the function cannot race against a concurrent kthread_stop() and exit. kthread_park() is more tricky, because its semantics are not well defined. It returns -ENOSYS if the thread exited but this can never happen and as Roman pointed out kthread_park() can obviously block forever if it would race with the exiting kthread. The usage of kthread_park() in cpuhp code (cpu.c, smpboot.c, stop_machine.c) is fine. It can never see an exiting/exited kthread, smpboot_destroy_threads() clears *ht->store, smpboot_park_thread() checks it is not NULL under the same smpboot_threads_lock. cpuhp_threads and cpu_stop_threads never exit, so other callers are fine too. But it has two more users: - watchdog_park_threads(): The code is actually correct, get_online_cpus() ensures that kthread_park() can't race with itself (note that kthread_park() can't handle this race correctly), but it should not use kthread_park() directly. - drivers/gpu/drm/amd/scheduler/gpu_scheduler.c should not use kthread_park() either. kthread_park() must not be called after amd_sched_fini() which does kthread_stop(), otherwise even to_live_kthread() is not safe because task_struct can be already freed and sched->thread can point to nowhere. The usage of kthread_park/unpark should either be restricted to core code which is properly protected against the exit race or made more robust so it is safe to use it in drivers. To catch eventual exit issues, add a WARN_ON(PF_EXITING) for now. Signed-off-by: Oleg Nesterov Acked-by: Peter Zijlstra (Intel) Reviewed-by: Thomas Gleixner Cc: Chunming Zhou Cc: Roman Pen Cc: Petr Mladek Cc: Andy Lutomirski Cc: Tejun Heo Cc: Andy Lutomirski Cc: Alex Deucher Cc: Andrew Morton Link: http://lkml.kernel.org/r/20161129175107.GA5339@redhat.com Signed-off-by: Thomas Gleixner --- kernel/kthread.c | 67 +++++++++++++++++------------------------------- 1 file changed, 23 insertions(+), 44 deletions(-) diff --git a/kernel/kthread.c b/kernel/kthread.c index 4dcbc8b5d6b6..01d27164e5b7 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -78,21 +78,6 @@ void free_kthread_struct(struct task_struct *k) kfree(to_kthread(k)); } -#define __to_kthread(vfork) \ - container_of(vfork, struct kthread, exited) - -/* - * TODO: kill it and use to_kthread(). But we still need the users - * like kthread_stop() which has to sync with the exiting kthread. - */ -static struct kthread *to_live_kthread(struct task_struct *k) -{ - struct completion *vfork = ACCESS_ONCE(k->vfork_done); - if (likely(vfork)) - return __to_kthread(vfork); - return NULL; -} - /** * kthread_should_stop - should this kthread return now? * @@ -441,8 +426,18 @@ struct task_struct *kthread_create_on_cpu(int (*threadfn)(void *data), return p; } -static void __kthread_unpark(struct task_struct *k, struct kthread *kthread) +/** + * kthread_unpark - unpark a thread created by kthread_create(). + * @k: thread created by kthread_create(). + * + * Sets kthread_should_park() for @k to return false, wakes it, and + * waits for it to return. If the thread is marked percpu then its + * bound to the cpu again. + */ +void kthread_unpark(struct task_struct *k) { + struct kthread *kthread = to_kthread(k); + clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags); /* * We clear the IS_PARKED bit here as we don't wait @@ -460,22 +455,6 @@ static void __kthread_unpark(struct task_struct *k, struct kthread *kthread) wake_up_state(k, TASK_PARKED); } } - -/** - * kthread_unpark - unpark a thread created by kthread_create(). - * @k: thread created by kthread_create(). - * - * Sets kthread_should_park() for @k to return false, wakes it, and - * waits for it to return. If the thread is marked percpu then its - * bound to the cpu again. - */ -void kthread_unpark(struct task_struct *k) -{ - struct kthread *kthread = to_live_kthread(k); - - if (kthread) - __kthread_unpark(k, kthread); -} EXPORT_SYMBOL_GPL(kthread_unpark); /** @@ -492,20 +471,20 @@ EXPORT_SYMBOL_GPL(kthread_unpark); */ int kthread_park(struct task_struct *k) { - struct kthread *kthread = to_live_kthread(k); - int ret = -ENOSYS; + struct kthread *kthread = to_kthread(k); - if (kthread) { - if (!test_bit(KTHREAD_IS_PARKED, &kthread->flags)) { - set_bit(KTHREAD_SHOULD_PARK, &kthread->flags); - if (k != current) { - wake_up_process(k); - wait_for_completion(&kthread->parked); - } + if (WARN_ON(k->flags & PF_EXITING)) + return -ENOSYS; + + if (!test_bit(KTHREAD_IS_PARKED, &kthread->flags)) { + set_bit(KTHREAD_SHOULD_PARK, &kthread->flags); + if (k != current) { + wake_up_process(k); + wait_for_completion(&kthread->parked); } - ret = 0; } - return ret; + + return 0; } EXPORT_SYMBOL_GPL(kthread_park); @@ -534,7 +513,7 @@ int kthread_stop(struct task_struct *k) get_task_struct(k); kthread = to_kthread(k); set_bit(KTHREAD_SHOULD_STOP, &kthread->flags); - __kthread_unpark(k, kthread); + kthread_unpark(k); wake_up_process(k); wait_for_completion(&kthread->exited); ret = k->exit_code;