From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from ozlabs.org (ozlabs.org [IPv6:2401:3900:2:1::2]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by lists.ozlabs.org (Postfix) with ESMTPS id E6B6F1A025B for ; Mon, 8 Dec 2014 14:27:02 +1100 (AEDT) From: Anton Blanchard To: torvalds@linux-foundation.org, akpm@linux-foundation.org, peterz@infradead.org, tglx@linutronix.de, mingo@redhat.com, rostedt@goodmis.org, tj@kernel.org, fengguang.wu@intel.com, rafael.j.wysocki@intel.com, yuyang.du@intel.com, lkp@01.org, yuanhan.liu@linux.intel.com, pjt@google.com, bsegall@google.com, daniel@numascale.com, subbaram@codeaurora.org, computersforpeace@gmail.com, sp@datera.io Subject: [PATCH] kthread: kthread_bind fails to enforce CPU affinity (fixes kernel BUG at kernel/smpboot.c:134!) Date: Mon, 8 Dec 2014 14:27:01 +1100 Message-Id: <1418009221-12719-1-git-send-email-anton@samba.org> Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , I have a busy ppc64le KVM box where guests sometimes hit the infamous "kernel BUG at kernel/smpboot.c:134!" issue during boot: BUG_ON(td->cpu != smp_processor_id()); Basically a per CPU hotplug thread scheduled on the wrong CPU. The oops output confirms it: CPU: 0 Comm: watchdog/130 The issue is in kthread_bind where we set the cpus_allowed mask, but do not touch task_thread_info(p)->cpu. The scheduler assumes the previously scheduled CPU is in the cpus_allowed mask, but in this case we are moving a thread to another CPU so it is not. We used to call set_task_cpu which sets task_thread_info(p)->cpu (in fact kthread_bind still has a comment suggesting this). That was removed in e2912009fb7b ("sched: Ensure set_task_cpu() is never called on blocked tasks"). Since we cannot call set_task_cpu (the task is in a sleeping state), just do an explicit set of task_thread_info(p)->cpu. Fixes: e2912009fb7b ("sched: Ensure set_task_cpu() is never called on blocked tasks") Cc: stable@vger.kernel.org Signed-off-by: Anton Blanchard --- kernel/kthread.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/kernel/kthread.c b/kernel/kthread.c index 10e489c..e40ab1d 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -327,13 +327,14 @@ EXPORT_SYMBOL(kthread_create_on_node); static void __kthread_bind(struct task_struct *p, unsigned int cpu, long state) { - /* Must have done schedule() in kthread() before we set_task_cpu */ + /* Must have done schedule() in kthread() before we change affinity */ if (!wait_task_inactive(p, state)) { WARN_ON(1); return; } /* It's safe because the task is inactive. */ do_set_cpus_allowed(p, cpumask_of(cpu)); + task_thread_info(p)->cpu = cpu; p->flags |= PF_NO_SETAFFINITY; } -- 2.1.0