Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752123AbaJTQ7t (ORCPT ); Mon, 20 Oct 2014 12:59:49 -0400 Received: from mx1.redhat.com ([209.132.183.28]:31873 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750935AbaJTQ7s (ORCPT ); Mon, 20 Oct 2014 12:59:48 -0400 Date: Mon, 20 Oct 2014 18:56:14 +0200 From: Oleg Nesterov To: Kirill Tkhai Cc: linux-kernel@vger.kernel.org, Peter Zijlstra , Ingo Molnar , Vladimir Davydov , Kirill Tkhai Subject: Re: [PATCH v3] sched/numa: fix unsafe get_task_struct() in task_numa_assign() Message-ID: <20141020165614.GA16373@redhat.com> References: <1413800145.19914.23.camel@tkhai> <20141020144757.GA10939@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20141020144757.GA10939@redhat.com> User-Agent: Mutt/1.5.18 (2008-05-17) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/20, Oleg Nesterov wrote: > > Again, perhaps we will need to change the lifetime rules for task_struct > anyway, if we have more problems like this. But until then this looks like > an overkill to me. Plus rq_curr_if_not_put() looks too subtle, and it is > not generic. Yes... otoh, perhaps we can do something more generic? Something like struct task_struct *xxx(struct task_struct **ptask) { struct task_struct *task; void *sighand; retry: task = ACCESS_ONCE(*ptask); if (!task) return NULL; if (IS_ENABLED(CONFIG_DEBUG_PAGEALLOC)) { if (probe_kernel_read(&sighand, &task->sighand, sizeof(sighand))) goto retry; } else { sighand = task->sighand; } if (!sighand) return NULL; /* * Pairs with atomic_dec_and_test() in put_task_struct(task). * If we have read the freed/reused memory, we must see that * the pointer was updated. */ smp_rmb(); if (task != ACCESS_ONCE(*ptask)) goto retry; return task; } task_numa_compare() can do cur = xxx(&rc->curr), but this helper can work with any "task_struct *" pointer assuming that somehow this pointer is cleared or changed before the final put_task_struct(). What do you think? Peter? Oleg. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/