Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752702Ab0KYLHQ (ORCPT ); Thu, 25 Nov 2010 06:07:16 -0500 Received: from mx1.redhat.com ([209.132.183.28]:25397 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752675Ab0KYLHP (ORCPT ); Thu, 25 Nov 2010 06:07:15 -0500 Date: Thu, 25 Nov 2010 12:00:13 +0100 From: Oleg Nesterov To: Peter Zijlstra Cc: Dave Jones , Linux Kernel , Andrew Morton , tglx , "Paul E. McKenney" Subject: Re: rcu_read_lock/unlock protect find_task_by_vpid call in posix_cpu_timer_create Message-ID: <20101125110013.GA19031@redhat.com> References: <20101125010948.GA1371@redhat.com> <1290674436.2072.562.camel@laptop> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1290674436.2072.562.camel@laptop> 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 Content-Length: 1742 Lines: 54 (add Sergey) On 11/25, Peter Zijlstra wrote: > > On Wed, 2010-11-24 at 20:09 -0500, Dave Jones wrote: > > --- a/kernel/posix-cpu-timers.c > > +++ b/kernel/posix-cpu-timers.c > > @@ -391,6 +391,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer) > > INIT_LIST_HEAD(&new_timer->it.cpu.entry); > > > > read_lock(&tasklist_lock); > > + rcu_read_lock(); > > if (CPUCLOCK_PERTHREAD(new_timer->it_clock)) { > > if (pid == 0) { > > p = current; > > @@ -414,6 +415,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer) > > } else { > > ret = -EINVAL; > > } > > + rcu_read_unlock(); > > read_unlock(&tasklist_lock); > > > > return ret; > > Do we still need the tasklist_lock in this case? No. posix-cpu-timer.c shouldn't use tasklist at all. But it is not completely trivial to remove it. In particular, this patch is not exactly right, we can't trust thread_group_leader() without tasklist. Sergey already sent the patch which removes tasklist from posix_cpu_timer_create() and posix_cpu_timer_create(), and iirc Thomas queued it. > Also, why is that think complaining, surely the tasklist_lock pins any > and all PID objects? The only problem is: if copy_process() fails, it does free_pid() lockless. This means, without rcu lock it is not safe to scan the rcu-protected lists. We can change copy_process() (in fact I sent the patch several years ago), but everybody think that find_pid/etc should always take rcu_read_lock() instead. I tend to agree. 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/