From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail.efficios.com ([167.114.142.141]:50447 "EHLO mail.efficios.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932271AbcKQLph (ORCPT ); Thu, 17 Nov 2016 06:45:37 -0500 Date: Thu, 17 Nov 2016 11:46:34 +0000 (UTC) From: Mathieu Desnoyers To: Lai Jiangshan Cc: "Paul E. McKenney" , linux-kernel , Josh Triplett , rostedt , stable Message-ID: <604945667.4957.1479383194368.JavaMail.zimbra@efficios.com> In-Reply-To: References: <1478190568-5829-1-git-send-email-mathieu.desnoyers@efficios.com> Subject: Re: [PATCH] Fix: disable sys_membarrier when nohz_full is enabled MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: 7bit Sender: stable-owner@vger.kernel.org List-ID: ----- On Nov 17, 2016, at 1:51 AM, Lai Jiangshan jiangshanlai@gmail.com wrote: > On Fri, Nov 4, 2016 at 12:29 AM, Mathieu Desnoyers > wrote: >> Userspace applications should be allowed to expect the membarrier system >> call with MEMBARRIER_CMD_SHARED command to issue memory barriers on >> nohz_full CPUs, but synchronize_sched() does not take those into >> account. >> >> Given that we do not want unrelated processes to be able to affect >> real-time sensitive nohz_full CPUs, simply return ENOSYS when membarrier >> is invoked on a kernel with enabled nohz_full CPUs. >> >> Signed-off-by: Mathieu Desnoyers >> CC: "Paul E. McKenney" >> CC: Josh Triplett >> CC: Steven Rostedt >> CC: Lai Jiangshan >> CC: [3.10+] >> --- >> kernel/membarrier.c | 4 ++++ >> 1 file changed, 4 insertions(+) >> >> diff --git a/kernel/membarrier.c b/kernel/membarrier.c >> index 536c727..9f9284f 100644 >> --- a/kernel/membarrier.c >> +++ b/kernel/membarrier.c >> @@ -16,6 +16,7 @@ >> >> #include >> #include >> +#include >> >> /* >> * Bitmask made from a "or" of all commands within enum membarrier_cmd, >> @@ -51,6 +52,9 @@ >> */ >> SYSCALL_DEFINE2(membarrier, int, cmd, int, flags) >> { >> + /* MEMBARRIER_CMD_SHARED is not compatible with nohz_full. */ >> + if (tick_nohz_full_enabled()) >> + return -ENOSYS; > > I guess this code needs to be moved down into the branch of > "case MEMBARRIER_CMD_SHARED" to match its comment. No, that would be unexpected from user-space. Either a system call is implemented or not, not "implemented for some parameters". We also want MEMBARRIER_CMD_QUERY to return -ENOSYS in this case, and all other parameter values to also return -ENOSYS (rather than -EINVAL). If a system call that returns successfully on CMD_QUERY or EINVAL, user-space may assume it will not have to handle ENOSYS in the next calls. > > Acked-by: Lai Jiangshan > > But I'm afraid, in the future, tick_nohz_full will become a default y > feature. thus it makes sys_membarrier() always disabled. we might > need a new MEMBARRIER_CMD_XXX to handle it? This may require that we send an IPI to nohz_full CPUs, which will disturb them real-time wise. Any better ideas ? Thanks, Mathieu > > thanks, > Lai -- Mathieu Desnoyers EfficiOS Inc. http://www.efficios.com