Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755257AbbHYKKl (ORCPT ); Tue, 25 Aug 2015 06:10:41 -0400 Received: from bombadil.infradead.org ([198.137.202.9]:37724 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752039AbbHYKKj (ORCPT ); Tue, 25 Aug 2015 06:10:39 -0400 Date: Tue, 25 Aug 2015 12:10:32 +0200 From: Peter Zijlstra To: Wanpeng Li Cc: Ingo Molnar , linux-kernel@vger.kernel.org Subject: Re: [PATCH] sched: fix tsk->pi_lock isn't held when do_set_cpus_allowed() Message-ID: <20150825101032.GI18673@twins.programming.kicks-ass.net> References: <20150825100527.GO16853@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20150825100527.GO16853@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2012-12-30) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1497 Lines: 43 On Tue, Aug 25, 2015 at 12:05:27PM +0200, Peter Zijlstra wrote: > On Tue, Aug 25, 2015 at 03:59:54PM +0800, Wanpeng Li wrote: > > +++ b/kernel/cpuset.c > > @@ -2376,8 +2376,12 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask) > > > > void cpuset_cpus_allowed_fallback(struct task_struct *tsk) > > { > > + unsigned long flags; > > + > > rcu_read_lock(); > > + raw_spin_lock_irqsave(&tsk->pi_lock, flags); > > do_set_cpus_allowed(tsk, task_cs(tsk)->effective_cpus); > > + raw_spin_lock_irqsave(&tsk->pi_lock, flags); > > rcu_read_unlock(); > > Aside from the double lock thing that was already pointed out, I think > this is wrong, because the select_task_rq() call can already have > pi_lock held. > > Taking it again would result in a deadlock. > > Consider for instance: > > try_to_wake_up() > raw_spin_lock_irqsave(->pi_lock) > select_task_rq() > select_ballback_rq() > cpuset_cpus_allowed_fallback() > raw_spin_lock_irqsave(->pi_lock) > > > The problem is with the migration path and should be fixed there. Another problem, migration_call() will have rq->lock held, so you're proposing to acquire pi_lock while holding rq->lock, this is an inversion from the regular nesting order. -- 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/