Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755077Ab0KCMzK (ORCPT ); Wed, 3 Nov 2010 08:55:10 -0400 Received: from mx1.redhat.com ([209.132.183.28]:4804 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753006Ab0KCMzH (ORCPT ); Wed, 3 Nov 2010 08:55:07 -0400 Date: Wed, 3 Nov 2010 13:48:35 +0100 From: Oleg Nesterov To: Sergey Senozhatsky Cc: Thomas Gleixner , Andrew Morton , Peter Zijlstra , Ingo Molnar , LKML , Stanislaw Gruszka Subject: Re: [PATCH] posix-cpu-timers: rcu_read_lock/unlock protect find_task_by_vpid call Message-ID: <20101103124835.GA604@redhat.com> References: <20101102135821.GA5964@swordfish.minsk.epam.com> <20101102160223.GC5964@swordfish.minsk.epam.com> <20101102183308.GA17720@redhat.com> <20101103105832.GA30053@swordfish.minsk.epam.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20101103105832.GA30053@swordfish.minsk.epam.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 Content-Length: 3253 Lines: 105 Damn. Sergey, Thomas, please wait a bit. Yes, I believe this patch is fine by itself. But looking into posix-cpu-timers.c again, I suspect that those "other problems with de_thread" I already mentioned are much more serious and need the urgent fix. I'll try to verify this a bit later today. In any case, I believe someone should find the time to audit/ rewrite posix-cpu-timers.c ;) On 11/03, Sergey Senozhatsky wrote: > > On (11/02/10 19:33), Oleg Nesterov wrote: > > > On Tue, 2 Nov 2010, Sergey Senozhatsky wrote: > > > > > > > > We can remove the tasklist_lock while at it. rcu_read_lock is enough. > > > > > > > > > Yes, I believe posix-cpu-timers.c shouldn't use tasklist at all, > > but it is not trivial to change this code. > > > >[..] > > > > I think this change is fine, but please note that thread_group_leader() > > check is not relaible without tasklist. If we race with de_thread() > > find_task_by_vpid() can find the new leader before it updates its > > ->group_leader. IOW, posix_cpu_timer_create() can fail when it shouldn't. > > > > Not that I think this really matters, posix_cpu_timer_create() has > > other problems with de_thread(). But perhaps it makes sense to > > change posix_cpu_timer_create() to use has_group_leader_pid() instead, > > just to make this code not look racy and avoid adding new problems. > > > > The real fix, I think, should change cpu_timer_list to use > > struct pid* instead of task_struct. > > > > Hello, > Using has_group_leader_pid instead of thread_group_leader, when tasklist_lock > is not aquired (check_clock and posix_cpu_timer_create). > > --- > > diff --git a/kernel/posix-cpu-timers.c b/kernel/posix-cpu-timers.c > index 6842eeb..05bb717 100644 > --- a/kernel/posix-cpu-timers.c > +++ b/kernel/posix-cpu-timers.c > @@ -37,13 +37,13 @@ static int check_clock(const clockid_t which_clock) > if (pid == 0) > return 0; > > - read_lock(&tasklist_lock); > + rcu_read_lock(); > p = find_task_by_vpid(pid); > if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ? > - same_thread_group(p, current) : thread_group_leader(p))) { > + same_thread_group(p, current) : has_group_leader_pid(p))) { > error = -EINVAL; > } > - read_unlock(&tasklist_lock); > + rcu_read_unlock(); > > return error; > } > @@ -390,7 +390,7 @@ 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; > @@ -404,7 +404,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer) > p = current->group_leader; > } else { > p = find_task_by_vpid(pid); > - if (p && !thread_group_leader(p)) > + if (p && !has_group_leader_pid(p)) > p = NULL; > } > } > @@ -414,7 +414,7 @@ int posix_cpu_timer_create(struct k_itimer *new_timer) > } else { > ret = -EINVAL; > } > - read_unlock(&tasklist_lock); > + rcu_read_unlock(); > > return ret; > } > > -- 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/