Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752897Ab0L2N3L (ORCPT ); Wed, 29 Dec 2010 08:29:11 -0500 Received: from mx1.redhat.com ([209.132.183.28]:48939 "EHLO mx1.redhat.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752437Ab0L2N3H (ORCPT ); Wed, 29 Dec 2010 08:29:07 -0500 Date: Wed, 29 Dec 2010 14:21:30 +0100 From: Oleg Nesterov To: Dario Faggioli Cc: Thomas Gleixner , linux-kernel , torbenh , john.stultz@linaro.org, roland@redhat.com, Ingo Molnar , Peter Zijlstra , Stanislaw Gruszka , Dhaval Giani , Randy Dunlap Subject: Re: [PATCH resend] Reading POSIX CPU timer from outside the process. Message-ID: <20101229132130.GA16349@redhat.com> References: <1293121303.3390.185.camel@Palantir> <1293533742.2899.1028.camel@Palantir> <20101228163829.GA26533@redhat.com> <1293572304.2899.1214.camel@Palantir> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <1293572304.2899.1214.camel@Palantir> 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: 3144 Lines: 87 On 12/28, Dario Faggioli wrote: > > On Tue, 2010-12-28 at 17:38 +0100, Oleg Nesterov wrote: > > > > rcu_read_lock(); > > > p = find_task_by_vpid(pid); > > > - if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ? > > > - same_thread_group(p, current) : has_group_leader_pid(p))) { > > > + if (!p || (CPUCLOCK_PERTHREAD(which_clock) && > > > + same_thread_group(p, current) && !has_group_leader_pid(p))) > > > error = -EINVAL; > > > - } > > > rcu_read_unlock(); > > > > How so? For example, with this change > > clock_getres(MAKE_THREAD_CPUCLOCK(pid_of_sub_thread)) won't work, no? > > > I tested all the clock_getres() calls that came to my mind (at least the > one that are possible from an userspace program), and they always worked > because of this (still in check_clock): > > const pid_t pid = CPUCLOCK_PID(which_clock); > > if (pid == 0) > return 0; > > Which triggers all the times, No, please note pid_of_sub_thread above. > The whole point was about, given the current implementation of > clock_getcpuclockid done by glibc, can we remove that "failed with > success" (showed in the changelog) thing and come up with some > meaningful clockid for that situation? It's more than possible for the > answer to be no!!! :-P I think we should change glibc if clock_getcpuclockid() doesn't work, please see below. > > I think, if we want to remove this limitation, we need something > > like the patch below. If it doesn't help, we should fix glibc. > > > > --- x/kernel/posix-cpu-timers.c > > +++ x/kernel/posix-cpu-timers.c > > @@ -39,10 +39,8 @@ static int check_clock(const clockid_t w > > > > rcu_read_lock(); > > p = find_task_by_vpid(pid); > > - if (!p || !(CPUCLOCK_PERTHREAD(which_clock) ? > > - same_thread_group(p, current) : has_group_leader_pid(p))) { > > + if (!p || !(CPUCLOCK_PERTHREAD(which_clock) || has_group_leader_pid(p))) > > error = -EINVAL; > > - } > > rcu_read_unlock(); > > > Which won't work because CPUCLOCK_PERTHREAD(which_clock) is always false > in this case. I guess, this is because glibc passes MAKE_PROCESS_CLOCK() id, right? But we shouldn't add the hacks to the kernel to hide the limitations in glibc. > BTW, again, I see your point, the fix might need to happen at glibc > level. I'll check that and come back if I find something interesting. Yes, please. BTW. What is the test-case? I am looking at http://gitorious.org/clockid, I guess it is clockid.c... You do not need clock_getcpuclockid() at all. In fact I do not really understand what this helper should actually do, probably it is only needed to validate the pid. You can simply use MAKE_THREAD_CPUCLOCK() to sample a single thread via clock_gettime(). IOW. Unless I missed something, with this patch, the only problem is that getcpuclockid() always assumes MAKE_PROCESS_CPUCLOCK(), I do not think this is the kernel problem. 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/