From mboxrd@z Thu Jan 1 00:00:00 1970 From: "Paul E. McKenney" Subject: Re: [PATCH RFC 02/26] task_work: Replace spin_unlock_wait() with lock/unlock pair Date: Fri, 30 Jun 2017 09:16:07 -0700 Message-ID: <20170630161607.GX2393@linux.vnet.ibm.com> References: <20170629235918.GA6445@linux.vnet.ibm.com> <1498780894-8253-2-git-send-email-paulmck@linux.vnet.ibm.com> <20170630110445.GA5123@redhat.com> <20170630125020.GU2393@linux.vnet.ibm.com> <20170630152010.GA6935@redhat.com> Reply-To: paulmck@linux.vnet.ibm.com Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: linux-kernel@vger.kernel.org, netfilter-devel@vger.kernel.org, netdev@vger.kernel.org, akpm@linux-foundation.org, mingo@redhat.com, dave@stgolabs.net, manfred@colorfullife.com, tj@kernel.org, arnd@arndb.de, linux-arch@vger.kernel.org, will.deacon@arm.com, peterz@infradead.org, stern@rowland.harvard.edu, parri.andrea@gmail.com, torvalds@linux-foundation.org To: Oleg Nesterov Return-path: Received: from mx0b-001b2d01.pphosted.com ([148.163.158.5]:34129 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-FAIL) by vger.kernel.org with ESMTP id S1751322AbdF3QQN (ORCPT ); Fri, 30 Jun 2017 12:16:13 -0400 Received: from pps.filterd (m0098420.ppops.net [127.0.0.1]) by mx0b-001b2d01.pphosted.com (8.16.0.20/8.16.0.20) with SMTP id v5UGDrq0035417 for ; Fri, 30 Jun 2017 12:16:12 -0400 Received: from e17.ny.us.ibm.com (e17.ny.us.ibm.com [129.33.205.207]) by mx0b-001b2d01.pphosted.com with ESMTP id 2bdqvsdkfe-1 (version=TLSv1.2 cipher=AES256-SHA bits=256 verify=NOT) for ; Fri, 30 Jun 2017 12:16:12 -0400 Received: from localhost by e17.ny.us.ibm.com with IBM ESMTP SMTP Gateway: Authorized Use Only! Violators will be prosecuted for from ; Fri, 30 Jun 2017 12:16:11 -0400 Content-Disposition: inline In-Reply-To: <20170630152010.GA6935@redhat.com> Sender: netfilter-devel-owner@vger.kernel.org List-ID: On Fri, Jun 30, 2017 at 05:20:10PM +0200, Oleg Nesterov wrote: > On 06/30, Paul E. McKenney wrote: > > > > > > + raw_spin_lock_irq(&task->pi_lock); > > > > + raw_spin_unlock_irq(&task->pi_lock); > > > > I agree that the spin_unlock_wait() implementations would avoid the > > deadlock with an acquisition from an interrupt handler, while also > > avoiding the need to momentarily disable interrupts. The ->pi_lock is > > a per-task lock, so I am assuming (perhaps naively) that contention is > > not a problem. So is the overhead of interrupt disabling likely to be > > noticeable here? > > I do not think the overhead will be noticeable in this particular case. > > But I am not sure I understand why do we want to unlock_wait. Yes I agree, > it has some problems, but still... > > The code above looks strange for me. If we are going to repeat this pattern > the perhaps we should add a helper for lock+unlock and name it unlock_wait2 ;) > > If not, we should probably change this code more: This looks -much- better than my patch! May I have your Signed-off-by? Thanx, Paul > --- a/kernel/task_work.c > +++ b/kernel/task_work.c > @@ -96,20 +96,16 @@ void task_work_run(void) > * work->func() can do task_work_add(), do not set > * work_exited unless the list is empty. > */ > + raw_spin_lock_irq(&task->pi_lock); > do { > work = READ_ONCE(task->task_works); > head = !work && (task->flags & PF_EXITING) ? > &work_exited : NULL; > } while (cmpxchg(&task->task_works, work, head) != work); > + raw_spin_unlock_irq(&task->pi_lock); > > if (!work) > break; > - /* > - * Synchronize with task_work_cancel(). It can't remove > - * the first entry == work, cmpxchg(task_works) should > - * fail, but it can play with *work and other entries. > - */ > - raw_spin_unlock_wait(&task->pi_lock); > > do { > next = work->next; > > performance-wise this is almost the same, and if we do not really care about > overhead we can simplify the code: this way it is obvious that we can't race > with task_work_cancel(). > > Oleg. >