From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org Received: from gabe.freedesktop.org (gabe.freedesktop.org [131.252.210.177]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id A0F79CCF9E3 for ; Thu, 30 Oct 2025 16:06:51 +0000 (UTC) Received: from gabe.freedesktop.org (localhost [127.0.0.1]) by gabe.freedesktop.org (Postfix) with ESMTP id 42A9E10E9FD; Thu, 30 Oct 2025 16:06:51 +0000 (UTC) Authentication-Results: gabe.freedesktop.org; dkim=pass (2048-bit key; unprotected) header.d=intel.com header.i=@intel.com header.b="FJJveKeO"; dkim-atps=neutral Received: from mgamail.intel.com (mgamail.intel.com [192.198.163.16]) by gabe.freedesktop.org (Postfix) with ESMTPS id 67CCF10E9FE for ; Thu, 30 Oct 2025 16:06:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=intel.com; i=@intel.com; q=dns/txt; s=Intel; t=1761840410; x=1793376410; h=date:from:to:cc:subject:message-id:references: mime-version:in-reply-to; bh=oKcEYwt2HUmd6KOn7O6kgLTGDoonjWfbWQswIlCsmCQ=; b=FJJveKeOlq2qbc4YUbEWRoWQQM3BSzSBsPHfpQSpSascVhgx3OGhtJn1 zT31koy/ge0L2uIzWcZBaGS9tTAYEysaG8OUT60f9uwb/lcG9VXMqMxV6 4ZkURrARsxV7ZC4Ob8kRTiKTU9WXdW+rUmxcgU59f5n8S+orI9Hq9OYOv P/EAH0GyB05z1N1FOtVcDbVZKGezyJ06WNrJOuC9DWn2CWcjrv4jUJa8S Y4itR+DPYucFYb0rn/BUJG/eTu4z2YvAtH8NU32ckGiHyklaBU3SUjg1h UaEeYT7D+MUAwDJkFe8Tr2m0avXKsAlCG2hC/EX+1iJJxVPzkI6u2r1Lg w==; X-CSE-ConnectionGUID: 2ZBMF+jvTfuhPu20RC2org== X-CSE-MsgGUID: q9KgPTvPTgi1U9zN4UOgYA== X-IronPort-AV: E=McAfee;i="6800,10657,11598"; a="51560314" X-IronPort-AV: E=Sophos;i="6.19,267,1754982000"; d="scan'208";a="51560314" Received: from orviesa009.jf.intel.com ([10.64.159.149]) by fmvoesa110.fm.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 30 Oct 2025 09:06:50 -0700 X-CSE-ConnectionGUID: gCGgG9URShy9y9l9/QKjiA== X-CSE-MsgGUID: dQvP+OtjRb2ouNt8/SilAA== X-ExtLoop1: 1 X-IronPort-AV: E=Sophos;i="6.19,267,1754982000"; d="scan'208";a="185653492" Received: from black.igk.intel.com ([10.91.253.5]) by orviesa009.jf.intel.com with ESMTP/TLS/ECDHE-RSA-AES256-GCM-SHA384; 30 Oct 2025 09:06:49 -0700 Date: Thu, 30 Oct 2025 17:06:46 +0100 From: Raag Jadav To: Rodrigo Vivi Cc: Lucas De Marchi , intel-xe@lists.freedesktop.org Subject: Re: [PATCH v3 8/8] drm/xe/gt_throttle: Avoid TOCTOU when monitoring reasons Message-ID: References: <20251029-gt-throttle-cri-v3-0-d1f5abbb8114@intel.com> <20251029-gt-throttle-cri-v3-8-d1f5abbb8114@intel.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: X-BeenThere: intel-xe@lists.freedesktop.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Intel Xe graphics driver List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: intel-xe-bounces@lists.freedesktop.org Sender: "Intel-xe" On Thu, Oct 30, 2025 at 11:47:03AM -0400, Rodrigo Vivi wrote: > On Thu, Oct 30, 2025 at 09:55:30AM -0500, Lucas De Marchi wrote: > > On Thu, Oct 30, 2025 at 10:53:36AM +0100, Raag Jadav wrote: > > > On Wed, Oct 29, 2025 at 04:45:10PM -0700, Lucas De Marchi wrote: > > > > It's currently not possible to safely monitor if there's throttling > > > > happening and what are the reasons. The approach of reading the status > > > > and then reading the reasons is not reliable as by the time sysadmin > > > > reads the reason, the throttling could not be happening anymore. > > > > > > > > Previous tentative to fix that[1] was breaking the ABI and potentially > > > > sysadmin's scripts. This takes a different approach of adding and > > > > documenting the additional attribute. It's still valuable, though > > > > redundant, to provide the simpler 0/1 interface. > > > > > > > > In order to avoid userspace knowledge on the bitmask meaning and to be > > > > able to maintain the kernel side in sync with possible changes in > > > > future, just walk the attribute group and check what are the masks that > > > > match the value read. > > > > > > > > [1] https://lore.kernel.org/intel-xe/20241025092238.167042-1-raag.jadav@intel.com/ > > > > > > ... > > > > > > > +static const struct attribute_group *get_platform_throttle_group(struct xe_device *xe); > > > > + > > > > +static ssize_t status_reasons_show(struct kobject *kobj, Semantically there's much of a 'status' here, so this could simply be 'reasons' (and same for the attribute name). > > > > + struct kobj_attribute *attr, char *buff) > > > > +{ > > > > + struct xe_gt *gt = throttle_to_gt(kobj); > > > > + struct xe_device *xe = gt_to_xe(gt); > > > > + const struct attribute_group *group; > > > > + struct attribute **pother; > > > > + ssize_t ret = 0; > > > > + u32 reasons; > > > > + > > > > + reasons = xe_gt_throttle_get_limit_reasons(gt); > > > > + group = get_platform_throttle_group(xe); > > > > + > > > > + for (pother = group->attrs; *pother; pother++) { > > > > + struct kobj_attribute *kattr = container_of(*pother, struct kobj_attribute, attr); > > > > + struct throttle_attribute *other_ta = kobj_attribute_to_throttle(kattr); > > > > + > > > > + if (other_ta->mask != U32_MAX && reasons & other_ta->mask) > > > > + ret += sysfs_emit_at(buff, ret, "%s ", (*pother)->name); > > > > > > Much better. > > > > > > > + } > > > > + > > > > + /* Drop extra space from last iteration above */ > > > > + if (ret) > > > > + ret--; > > > > + > > > > + ret += sysfs_emit_at(buff, ret, "\n"); > > > > > > I went through the documentation again and I couldn't find any rules > > > related to empty files or whether it is allowed (just thinking out > > > loud about no throttling cases). > > > > do you mean if "empty" files are allowed in sysfs? I don't think there's > > any problem with that. It's also not empty, it has a newline there ;) > > alternatively we could print the entire reg in hex format? > But I prefer the text line in this patch. > > Nothing against the 'empty' file with or without the new-line, > but perhaps we could consider to track that in the loop > and if none is add we print > > if (ret) > ret--; > else > sysfs_emit_at(buff, ret, "none"); > > and document that above... +1. PS: A good read[1] if anyone's interested. [1] https://lwn.net/Articles/378884/ Raag