From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752063Ab3K3OGk (ORCPT ); Sat, 30 Nov 2013 09:06:40 -0500 Received: from mail-bk0-f49.google.com ([209.85.214.49]:58933 "EHLO mail-bk0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751585Ab3K3OGh (ORCPT ); Sat, 30 Nov 2013 09:06:37 -0500 Date: Sat, 30 Nov 2013 15:06:33 +0100 From: Ingo Molnar To: Peter Zijlstra Cc: Juri Lelli , tglx@linutronix.de, mingo@redhat.com, rostedt@goodmis.org, oleg@redhat.com, fweisbec@gmail.com, darren@dvhart.com, johan.eker@ericsson.com, p.faure@akatech.ch, linux-kernel@vger.kernel.org, claudio@evidence.eu.com, michael@amarulasolutions.com, fchecconi@gmail.com, tommaso.cucinotta@sssup.it, nicola.manica@disi.unitn.it, luca.abeni@unitn.it, dhaval.giani@gmail.com, hgu1972@gmail.com, paulmck@linux.vnet.ibm.com, raistlin@linux.it, insop.song@gmail.com, liming.wang@windriver.com, jkacur@redhat.com, harald.gustafsson@ericsson.com, vincent.guittot@linaro.org, bruce.ashfield@windriver.com, Andrew Morton , Linus Torvalds Subject: Re: [PATCH 02/14] sched: add extended scheduling interface. (new ABI) Message-ID: <20131130140633.GB19027@gmail.com> References: <1383831828-15501-1-git-send-email-juri.lelli@gmail.com> <1383831828-15501-3-git-send-email-juri.lelli@gmail.com> <20131127132354.GA18422@gmail.com> <20131127133027.GE10022@twins.programming.kicks-ass.net> <20131127140143.GB24403@gmail.com> <20131127141334.GA13532@twins.programming.kicks-ass.net> <20131127141721.GA24979@gmail.com> <5297257B.7090200@gmail.com> <20131128112858.GK10022@twins.programming.kicks-ass.net> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20131128112858.GK10022@twins.programming.kicks-ass.net> User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org * Peter Zijlstra wrote: > On Thu, Nov 28, 2013 at 12:14:03PM +0100, Juri Lelli wrote: > > +SYSCALL_DEFINE2(sched_getattr, pid_t, pid, struct sched_attr __user *, attr) > > { > > - struct sched_param2 lp; > > + struct sched_attr lp; > > struct task_struct *p; > > int retval; > > > > - if (!param2 || pid < 0) > > + if (!attr || pid < 0) > > return -EINVAL; > > > > + memset(&lp, 0, sizeof(struct sched_attr)); > > + > > rcu_read_lock(); > > p = find_process_by_pid(pid); > > retval = -ESRCH; > > @@ -3427,7 +3495,7 @@ SYSCALL_DEFINE2(sched_getparam2, pid_t, pid, struct sched_param2 __user *, param > > lp.sched_priority = p->rt_priority; > > rcu_read_unlock(); > > > > - retval = copy_to_user(param2, &lp, sizeof(lp)) ? -EFAULT : 0; > > + retval = copy_to_user(attr, &lp, sizeof(lp)) ? -EFAULT : 0; > > return retval; > > > > out_unlock: > > > So this side needs a bit more care; suppose the kernel has a larger attr > than userspace knows about. > > What would make more sense; add another syscall argument with the > userspace sizeof(struct sched_attr), or expect userspace to initialize > attr->size to the right value before calling sched_getattr() ? > > To me the extra argument makes more sense; that is: > > struct sched_attr attr; > > ret = sched_getattr(0, &attr, sizeof(attr)); > > seems like a saner thing than: > > struct sched_attr attr = { .size = sizeof(attr), }; > > ret = sched_getattr(0, &attr); > > Mostly because the former has a clear separation between input and > output arguments, whereas for the second form the attr argument is > both input and output. > > Ingo? I suppose so - in the sys_perf_event_open() case we ran out of arguments, so attr::size was the only sane way to do it. [ Btw., perf events side note: for completeness and for debugging we probably want to add a sys_perf_event_get() method as well, to recover the attributes of an existing event. Unidirectional APIs are not very nice. ] Thanks, Ingo