Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752269AbdFZWSz (ORCPT ); Mon, 26 Jun 2017 18:18:55 -0400 Received: from smtp.codeaurora.org ([198.145.29.96]:36066 "EHLO smtp.codeaurora.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752161AbdFZWSO (ORCPT ); Mon, 26 Jun 2017 18:18:14 -0400 DMARC-Filter: OpenDMARC Filter v1.3.2 smtp.codeaurora.org EFA5B60E0E Authentication-Results: pdx-caf-mail.web.codeaurora.org; dmarc=none (p=none dis=none) header.from=codeaurora.org Authentication-Results: pdx-caf-mail.web.codeaurora.org; spf=none smtp.mailfrom=markivx@codeaurora.org From: Vikram Mulukutla To: rusty@rustcorp.com.au, tj@kernel.org, tglx@linuxtronix.de, akpm@linux-foundation.org Cc: linux-kernel@vger.kernel.org, stable@vger.kernel.org, Vikram Mulukutla Subject: [PATCH] kthread: Atomically set completion and perform dequeue in __kthread_parkme Date: Mon, 26 Jun 2017 15:18:03 -0700 Message-Id: <1498515483-12743-1-git-send-email-markivx@codeaurora.org> X-Mailer: git-send-email 1.9.1 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 4326 Lines: 89 kthread_park waits for the target kthread to park itself with __kthread_parkme using a completion variable. __kthread_parkme - which is invoked by the target kthread - sets the completion variable before calling schedule() to voluntarily get itself off of the runqueue. This causes an interesting race in the hotplug path. takedown_cpu() invoked for CPU_X attempts to park the cpuhp/X hotplug kthread before running the stopper thread on CPU_X. kthread_unpark doesn't guarantee that cpuhp/X is off of X's runqueue, only that the thread has executed __kthread_parkme and set the completion. cpuhp/X may have been preempted out before calling schedule() to voluntarily sleep. takedown_cpu proceeds to run the stopper thread on CPU_X which promptly migrates off the still-on-rq cpuhp/X thread to another cpu CPU_Y, setting its affinity mask to something other than CPU_X alone. This is OK - cpuhp/X may finally get itself off of CPU_Y's runqueue at some later point. But if that doesn't happen (for example, if there's an RT thread on CPU_Y), the kthread_unpark in a subsequent cpu_up call for CPU_X will race with the still-on-rq condition. Even now we're functionally OK because there is a wait_task_inactive in the kthread_unpark(), BUT the following happens: [ 12.472745] BUG: scheduling while atomic: swapper/7/0/0x00000002 [ 12.472749] Modules linked in: [ 12.472756] CPU: 7 PID: 0 Comm: swapper/7 Not tainted 4.9.32-perf+ #680 [ 12.472758] Hardware name: XXXXX [ 12.472760] Call trace: [ 12.472773] [] dump_backtrace+0x0/0x198 [ 12.472777] [] show_stack+0x14/0x1c [ 12.472781] [] dump_stack+0x8c/0xac [ 12.472786] [] __schedule_bug+0x54/0x70 [ 12.472792] [] __schedule+0x6b4/0x928 [ 12.472794] [] schedule+0x3c/0xa0 [ 12.472797] [] schedule_hrtimeout_range_clock+0x80/0xec [ 12.472799] [] schedule_hrtimeout+0x18/0x20 [ 12.472803] [] wait_task_inactive+0x1a0/0x1a4 [ 12.472806] [] __kthread_bind_mask+0x20/0x7c [ 12.472809] [] __kthread_bind+0x28/0x30 [ 12.472811] [] __kthread_unpark+0x5c/0x60 [ 12.472814] [] kthread_unpark+0x24/0x2c [ 12.472818] [] cpuhp_online_idle+0x50/0x90 [ 12.472822] [] cpu_startup_entry+0x3c/0x1d4 [ 12.472824] [] secondary_start_kernel+0x164/0x1b4 Since the kthread_unpark is invoked from a preemption-disabled context, wait_task_inactive's action of invoking schedule is invalid, causing the splat. Note that kthread_bind_mask is correctly attempting to re-set the affinity mask since cpuhp is a per-cpu smpboot thread. Instead of adding an expensive wait_task_inactive inside kthread_park() or trying to muck with the hotplug code, let's just ensure that the completion variable and the schedule happen atomically inside __kthread_parkme. This focuses the fix to the hotplug requirement alone, and removes the unnecessary migration of cpuhp/X. Signed-off-by: Vikram Mulukutla --- kernel/kthread.c | 13 ++++++++++++- 1 file changed, 12 insertions(+), 1 deletion(-) diff --git a/kernel/kthread.c b/kernel/kthread.c index 26db528..7ad3354 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -171,9 +171,20 @@ static void __kthread_parkme(struct kthread *self) { __set_current_state(TASK_PARKED); while (test_bit(KTHREAD_SHOULD_PARK, &self->flags)) { + /* + * Why the preempt_disable? + * Hotplug needs to ensure that 'self' is off of the runqueue + * as well, before scheduling the stopper thread that will + * migrate tasks off of the runqeue that 'self' was running on. + * This avoids unnecessary migration work and also ensures that + * kthread_unpark in the cpu_up path doesn't race with + * __kthread_parkme. + */ + preempt_disable(); if (!test_and_set_bit(KTHREAD_IS_PARKED, &self->flags)) complete(&self->parked); - schedule(); + schedule_preempt_disabled(); + preempt_enable(); __set_current_state(TASK_PARKED); } clear_bit(KTHREAD_IS_PARKED, &self->flags); -- The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project