From mboxrd@z Thu Jan 1 00:00:00 1970 From: Dave Hansen Date: Tue, 28 Oct 2014 17:43:48 +0000 Subject: Re: [PATCH v9 05/12] x86, mpx: on-demand kernel allocation of bounds tables Message-Id: <544FD5D4.4090404@intel.com> List-Id: References: <1413088915-13428-1-git-send-email-qiaowei.ren@intel.com> <1413088915-13428-6-git-send-email-qiaowei.ren@intel.com> In-Reply-To: MIME-Version: 1.0 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit To: Thomas Gleixner , Qiaowei Ren Cc: "H. Peter Anvin" , Ingo Molnar , x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-ia64@vger.kernel.org, linux-mips@linux-mips.org On 10/24/2014 05:08 AM, Thomas Gleixner wrote: > On Sun, 12 Oct 2014, Qiaowei Ren wrote: >> + /* >> + * Go poke the address of the new bounds table in to the >> + * bounds directory entry out in userspace memory. Note: >> + * we may race with another CPU instantiating the same table. >> + * In that case the cmpxchg will see an unexpected >> + * 'actual_old_val'. >> + */ >> + ret = user_atomic_cmpxchg_inatomic(&actual_old_val, bd_entry, >> + expected_old_val, bt_addr); > > This is fully preemptible non-atomic context, right? > > So this wants a proper comment, why using > user_atomic_cmpxchg_inatomic() is the right thing to do here. Hey Thomas, How's this for a new comment? Does this cover the points you think need clarified? == The kernel has allocated a bounds table and needs to point the (userspace-allocated) directory to it. The directory entry is the *only* place we track that this table was allocated, so we essentially use it instead of an kernel data structure for synchronization. A copy_to_user()-style function would not give us the atomicity that we need. If two threads race to instantiate a table, the cmpxchg ensures we know which one lost the race and that the loser frees the table that they just allocated. From mboxrd@z Thu Jan 1 00:00:00 1970 Received: with ECARTIS (v1.0.0; list linux-mips); Tue, 28 Oct 2014 18:43:59 +0100 (CET) Received: from mga09.intel.com ([134.134.136.24]:53466 "EHLO mga09.intel.com" rhost-flags-OK-OK-OK-OK) by eddie.linux-mips.org with ESMTP id S27011669AbaJ1Rn6IYgZT (ORCPT ); Tue, 28 Oct 2014 18:43:58 +0100 Received: from orsmga001.jf.intel.com ([10.7.209.18]) by orsmga102.jf.intel.com with ESMTP; 28 Oct 2014 10:42:27 -0700 X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="5.04,804,1406617200"; d="scan'208";a="597816713" Received: from ray.jf.intel.com (HELO [10.7.199.163]) ([10.7.199.163]) by orsmga001.jf.intel.com with ESMTP; 28 Oct 2014 10:43:48 -0700 Message-ID: <544FD5D4.4090404@intel.com> Date: Tue, 28 Oct 2014 10:43:48 -0700 From: Dave Hansen User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:31.0) Gecko/20100101 Thunderbird/31.1.2 MIME-Version: 1.0 To: Thomas Gleixner , Qiaowei Ren CC: "H. Peter Anvin" , Ingo Molnar , x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-ia64@vger.kernel.org, linux-mips@linux-mips.org Subject: Re: [PATCH v9 05/12] x86, mpx: on-demand kernel allocation of bounds tables References: <1413088915-13428-1-git-send-email-qiaowei.ren@intel.com> <1413088915-13428-6-git-send-email-qiaowei.ren@intel.com> In-Reply-To: Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Return-Path: X-Envelope-To: <"|/home/ecartis/ecartis -s linux-mips"> (uid 0) X-Orcpt: rfc822;linux-mips@linux-mips.org Original-Recipient: rfc822;linux-mips@linux-mips.org X-archive-position: 43646 X-ecartis-version: Ecartis v1.0.0 Sender: linux-mips-bounce@linux-mips.org Errors-to: linux-mips-bounce@linux-mips.org X-original-sender: dave.hansen@intel.com Precedence: bulk List-help: List-unsubscribe: List-software: Ecartis version 1.0.0 List-Id: linux-mips X-List-ID: linux-mips List-subscribe: List-owner: List-post: List-archive: X-list: linux-mips On 10/24/2014 05:08 AM, Thomas Gleixner wrote: > On Sun, 12 Oct 2014, Qiaowei Ren wrote: >> + /* >> + * Go poke the address of the new bounds table in to the >> + * bounds directory entry out in userspace memory. Note: >> + * we may race with another CPU instantiating the same table. >> + * In that case the cmpxchg will see an unexpected >> + * 'actual_old_val'. >> + */ >> + ret = user_atomic_cmpxchg_inatomic(&actual_old_val, bd_entry, >> + expected_old_val, bt_addr); > > This is fully preemptible non-atomic context, right? > > So this wants a proper comment, why using > user_atomic_cmpxchg_inatomic() is the right thing to do here. Hey Thomas, How's this for a new comment? Does this cover the points you think need clarified? ==== The kernel has allocated a bounds table and needs to point the (userspace-allocated) directory to it. The directory entry is the *only* place we track that this table was allocated, so we essentially use it instead of an kernel data structure for synchronization. A copy_to_user()-style function would not give us the atomicity that we need. If two threads race to instantiate a table, the cmpxchg ensures we know which one lost the race and that the loser frees the table that they just allocated. From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-pa0-f49.google.com (mail-pa0-f49.google.com [209.85.220.49]) by kanga.kvack.org (Postfix) with ESMTP id 3631A900021 for ; Tue, 28 Oct 2014 13:43:52 -0400 (EDT) Received: by mail-pa0-f49.google.com with SMTP id lj1so1224171pab.22 for ; Tue, 28 Oct 2014 10:43:51 -0700 (PDT) Received: from mga09.intel.com (mga09.intel.com. [134.134.136.24]) by mx.google.com with ESMTP id qd9si1837590pdb.221.2014.10.28.10.43.50 for ; Tue, 28 Oct 2014 10:43:51 -0700 (PDT) Message-ID: <544FD5D4.4090404@intel.com> Date: Tue, 28 Oct 2014 10:43:48 -0700 From: Dave Hansen MIME-Version: 1.0 Subject: Re: [PATCH v9 05/12] x86, mpx: on-demand kernel allocation of bounds tables References: <1413088915-13428-1-git-send-email-qiaowei.ren@intel.com> <1413088915-13428-6-git-send-email-qiaowei.ren@intel.com> In-Reply-To: Content-Type: text/plain; charset=windows-1252 Content-Transfer-Encoding: 7bit Sender: owner-linux-mm@kvack.org List-ID: To: Thomas Gleixner , Qiaowei Ren Cc: "H. Peter Anvin" , Ingo Molnar , x86@kernel.org, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-ia64@vger.kernel.org, linux-mips@linux-mips.org On 10/24/2014 05:08 AM, Thomas Gleixner wrote: > On Sun, 12 Oct 2014, Qiaowei Ren wrote: >> + /* >> + * Go poke the address of the new bounds table in to the >> + * bounds directory entry out in userspace memory. Note: >> + * we may race with another CPU instantiating the same table. >> + * In that case the cmpxchg will see an unexpected >> + * 'actual_old_val'. >> + */ >> + ret = user_atomic_cmpxchg_inatomic(&actual_old_val, bd_entry, >> + expected_old_val, bt_addr); > > This is fully preemptible non-atomic context, right? > > So this wants a proper comment, why using > user_atomic_cmpxchg_inatomic() is the right thing to do here. Hey Thomas, How's this for a new comment? Does this cover the points you think need clarified? ==== The kernel has allocated a bounds table and needs to point the (userspace-allocated) directory to it. The directory entry is the *only* place we track that this table was allocated, so we essentially use it instead of an kernel data structure for synchronization. A copy_to_user()-style function would not give us the atomicity that we need. If two threads race to instantiate a table, the cmpxchg ensures we know which one lost the race and that the loser frees the table that they just allocated. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@kvack.org. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: email@kvack.org