From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752537AbeFEUNf (ORCPT ); Tue, 5 Jun 2018 16:13:35 -0400 Received: from merlin.infradead.org ([205.233.59.134]:44854 "EHLO merlin.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752475AbeFEUNe (ORCPT ); Tue, 5 Jun 2018 16:13:34 -0400 Date: Tue, 5 Jun 2018 22:13:16 +0200 From: Peter Zijlstra To: Oleg Nesterov Cc: "Kohli, Gaurav" , tglx@linutronix.de, mpe@ellerman.id.au, mingo@kernel.org, bigeasy@linutronix.de, linux-kernel@vger.kernel.org, linux-arm-msm@vger.kernel.org, Neeraj Upadhyay , Will Deacon Subject: Re: [PATCH v1] kthread/smpboot: Serialize kthread parking against wakeup Message-ID: <20180605201316.GZ12198@hirez.programming.kicks-ass.net> References: <9b289790-9b3a-73bd-7166-bf39f32cefd8@codeaurora.org> <20180502082011.GB12180@hirez.programming.kicks-ass.net> <830d7225-af90-a55a-991a-bb2023d538f1@codeaurora.org> <55221a5b-dd52-3359-f582-86830dd9f205@codeaurora.org> <20180605150841.GA24053@redhat.com> <20180605152212.GY12180@hirez.programming.kicks-ass.net> <20180605154053.GB12235@hirez.programming.kicks-ass.net> <20180605163515.GB24053@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180605163515.GB24053@redhat.com> User-Agent: Mutt/1.9.5 (2018-04-13) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Tue, Jun 05, 2018 at 06:35:16PM +0200, Oleg Nesterov wrote: > Yes, but this won't fix the race decribed by Kohli... Clearly I'm not going strong today... and yes, looking at that again I didn't address that. > Plus this complicates the schedule() paths for the very special case I checked, we already spilled @preempt onto the stack, context_switch() is inlined so the extra argument is a no-op, if finish_task_switch() also gets inlined (possible, could force it) the only additional cost is the @preempt load from stack in the slow path. If it doesn't get inlined we get the additional function call overhead, which should be minimal. But yes, I wasn't a fan either. > and to me it seems that all this kthread_park/unpark logic needs some > serious cleanups... I really hated how even with TASK_PARKED special the smpboot thread could still get migrated, which is why I moved the completion. Do you have any saner suggestions? Humm, I suppose we could do wait_task_inactive() after wait_for_completion(). I absolutely abhor wait_task_inactive(), but I remember us both failing to fix that at least twice :/ Also, I think we still need TASK_PARKED as a special state for that. How's the below? --- include/linux/kthread.h | 1 - include/linux/sched.h | 2 +- kernel/kthread.c | 32 +++++++++++++++++++++++++------- kernel/sched/core.c | 31 +++++++++++-------------------- 4 files changed, 37 insertions(+), 29 deletions(-) diff --git a/include/linux/kthread.h b/include/linux/kthread.h index 2803264c512f..c1961761311d 100644 --- a/include/linux/kthread.h +++ b/include/linux/kthread.h @@ -62,7 +62,6 @@ void *kthread_probe_data(struct task_struct *k); int kthread_park(struct task_struct *k); void kthread_unpark(struct task_struct *k); void kthread_parkme(void); -void kthread_park_complete(struct task_struct *k); int kthreadd(void *unused); extern struct task_struct *kthreadd_task; diff --git a/include/linux/sched.h b/include/linux/sched.h index 14e4f9c12337..4e32c1cc7794 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -117,7 +117,7 @@ struct task_group; * the comment with set_special_state(). */ #define is_special_task_state(state) \ - ((state) & (__TASK_STOPPED | __TASK_TRACED | TASK_DEAD)) + ((state) & (__TASK_STOPPED | __TASK_TRACED | TASK_PARKED | TASK_DEAD)) #define __set_current_state(state_value) \ do { \ diff --git a/kernel/kthread.c b/kernel/kthread.c index 481951bf091d..8f66a3dc767a 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -177,12 +177,24 @@ void *kthread_probe_data(struct task_struct *task) static void __kthread_parkme(struct kthread *self) { for (;;) { - set_current_state(TASK_PARKED); + /* + * TASK_PARKED is a special state; we must serialize against + * possible pending wakeups to avoid store-store collisions on + * task->state. + * + * Such a collision might possibly result in the task state + * changin from TASK_PARKED and us failing the + * wait_task_inactive() in kthread_park(). + */ + set_special_state(TASK_PARKED); if (!test_bit(KTHREAD_SHOULD_PARK, &self->flags)) break; + + complete_all(&self->parked); schedule(); } __set_current_state(TASK_RUNNING); + reinit_completion(&self->parked); } void kthread_parkme(void) @@ -191,11 +203,6 @@ void kthread_parkme(void) } EXPORT_SYMBOL_GPL(kthread_parkme); -void kthread_park_complete(struct task_struct *k) -{ - complete_all(&to_kthread(k)->parked); -} - static int kthread(void *_create) { /* Copy data: it's on kthread's stack */ @@ -459,8 +466,10 @@ void kthread_unpark(struct task_struct *k) if (test_bit(KTHREAD_IS_PER_CPU, &kthread->flags)) __kthread_bind(k, kthread->cpu, TASK_PARKED); - reinit_completion(&kthread->parked); clear_bit(KTHREAD_SHOULD_PARK, &kthread->flags); + /* + * __kthread_parkme() will either see !SHOULD_PARK or get the wakeup. + */ wake_up_state(k, TASK_PARKED); } EXPORT_SYMBOL_GPL(kthread_unpark); @@ -487,7 +496,16 @@ int kthread_park(struct task_struct *k) set_bit(KTHREAD_SHOULD_PARK, &kthread->flags); if (k != current) { wake_up_process(k); + /* + * Wait for __kthread_parkme() to complete(), this means we + * _will_ have TASK_PARKED and are about to call schedule(). + */ wait_for_completion(&kthread->parked); + /* + * Now wait for that schedule() to complete and the task to + * get scheduled out. + */ + WARN_ON_ONCE(!wait_task_inactive(p, TASK_PARKED)); } return 0; diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 8d59b259af4a..cf72c4eed7da 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -7,7 +7,6 @@ */ #include "sched.h" -#include #include #include @@ -2701,28 +2700,20 @@ static struct rq *finish_task_switch(struct task_struct *prev) membarrier_mm_sync_core_before_usermode(mm); mmdrop(mm); } - if (unlikely(prev_state & (TASK_DEAD|TASK_PARKED))) { - switch (prev_state) { - case TASK_DEAD: - if (prev->sched_class->task_dead) - prev->sched_class->task_dead(prev); + if (unlikely(prev_state == TASK_DEAD)) { + if (prev->sched_class->task_dead) + prev->sched_class->task_dead(prev); - /* - * Remove function-return probe instances associated with this - * task and put them back on the free list. - */ - kprobe_flush_task(prev); - - /* Task is done with its stack. */ - put_task_stack(prev); + /* + * Remove function-return probe instances associated with this + * task and put them back on the free list. + */ + kprobe_flush_task(prev); - put_task_struct(prev); - break; + /* Task is done with its stack. */ + put_task_stack(prev); - case TASK_PARKED: - kthread_park_complete(prev); - break; - } + put_task_struct(prev); } tick_nohz_task_switch();