From mboxrd@z Thu Jan 1 00:00:00 1970 From: Frederic Weisbecker Subject: Re: [PATCH v4 2/5] x86,entry: Only call user_exit if TIF_NOHZ Date: Wed, 30 Jul 2014 18:43:46 +0200 Message-ID: <20140730164344.GA27954@localhost.localdomain> References: <7123b2489cc5d1d5abb7bcf1364ca729cab3e6ca.1406604806.git.luto@amacapital.net> <20140729193232.GA8153@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Return-path: Content-Disposition: inline In-Reply-To: <20140729193232.GA8153@redhat.com> Sender: linux-security-module-owner@vger.kernel.org To: Oleg Nesterov Cc: Andy Lutomirski , linux-kernel@vger.kernel.org, Kees Cook , Will Drewry , x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-mips@linux-mips.org, linux-arch@vger.kernel.org, linux-security-module@vger.kernel.org, Alexei Starovoitov , hpa@zytor.com List-Id: linux-arch.vger.kernel.org On Tue, Jul 29, 2014 at 09:32:32PM +0200, Oleg Nesterov wrote: > On 07/28, Andy Lutomirski wrote: > > > > @@ -1449,7 +1449,12 @@ long syscall_trace_enter(struct pt_regs *regs) > > { > > long ret = 0; > > > > - user_exit(); > > + /* > > + * If TIF_NOHZ is set, we are required to call user_exit() before > > + * doing anything that could touch RCU. > > + */ > > + if (test_thread_flag(TIF_NOHZ)) > > + user_exit(); > > Personally I still think this change just adds more confusion, but I leave > this to you and Frederic. > > It is not that "If TIF_NOHZ is set, we are required to call user_exit()", we > need to call user_exit() just because we enter the kernel. TIF_NOHZ is just > the implementation detail which triggers this slow path. > > At least it should be correct, unless I am confused even more than I think. Agreed, Perhaps the confusion is on the syscall_trace_enter() name which suggests this is only about tracing? syscall_slowpath_enter() could be an alternative. But that's still tracing in a general sense so... From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-wi0-f177.google.com ([209.85.212.177]:62623 "EHLO mail-wi0-f177.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755209AbaG3Qnx (ORCPT ); Wed, 30 Jul 2014 12:43:53 -0400 Date: Wed, 30 Jul 2014 18:43:46 +0200 From: Frederic Weisbecker Subject: Re: [PATCH v4 2/5] x86,entry: Only call user_exit if TIF_NOHZ Message-ID: <20140730164344.GA27954@localhost.localdomain> References: <7123b2489cc5d1d5abb7bcf1364ca729cab3e6ca.1406604806.git.luto@amacapital.net> <20140729193232.GA8153@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20140729193232.GA8153@redhat.com> Sender: linux-arch-owner@vger.kernel.org List-ID: To: Oleg Nesterov Cc: Andy Lutomirski , linux-kernel@vger.kernel.org, Kees Cook , Will Drewry , x86@kernel.org, linux-arm-kernel@lists.infradead.org, linux-mips@linux-mips.org, linux-arch@vger.kernel.org, linux-security-module@vger.kernel.org, Alexei Starovoitov , hpa@zytor.com Message-ID: <20140730164346.auAXni0hvcntuO_Xc_0LaU-bOHHNJ8KBzTDFRQOzXkw@z> On Tue, Jul 29, 2014 at 09:32:32PM +0200, Oleg Nesterov wrote: > On 07/28, Andy Lutomirski wrote: > > > > @@ -1449,7 +1449,12 @@ long syscall_trace_enter(struct pt_regs *regs) > > { > > long ret = 0; > > > > - user_exit(); > > + /* > > + * If TIF_NOHZ is set, we are required to call user_exit() before > > + * doing anything that could touch RCU. > > + */ > > + if (test_thread_flag(TIF_NOHZ)) > > + user_exit(); > > Personally I still think this change just adds more confusion, but I leave > this to you and Frederic. > > It is not that "If TIF_NOHZ is set, we are required to call user_exit()", we > need to call user_exit() just because we enter the kernel. TIF_NOHZ is just > the implementation detail which triggers this slow path. > > At least it should be correct, unless I am confused even more than I think. Agreed, Perhaps the confusion is on the syscall_trace_enter() name which suggests this is only about tracing? syscall_slowpath_enter() could be an alternative. But that's still tracing in a general sense so...