From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from frasgout12.his.huawei.com (frasgout12.his.huawei.com [14.137.139.154]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id B4EC2180A80; Mon, 30 Sep 2024 08:58:15 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=14.137.139.154 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727686697; cv=none; b=DvudLOkSm1XA43EVDKWYYFjOfDIyY0jP0WP7kGNYqrZBwWt7SVvEqdhPsCQYXQbtDmUVYqP2CFlK8yxBYlJ2HWCpO/8AWoz7f5Ovt+iBRPsPiGonp+rygojWiOtwEWCRHrT6yKjJTxLURo/NxO4cMnJGNYJO4GwJcqS7Fw8z48A= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1727686697; c=relaxed/simple; bh=pK9BBmAz7pGForOQWbZmxWWr5mRMu1MTSBFFRLgbCTw=; h=Message-ID:Date:MIME-Version:Subject:To:Cc:References:From: In-Reply-To:Content-Type; b=VsK9JJWno8MUdM1v59lKeBdr/ycX7Ai5bXJwZHimikEXIPhC6MR6F7ap/wU35c8G0UpqcOlw8RHT8ptBp4sAI8+Xi6fgdFZ1lE1KrdCt8Im76QD3DYV6B2PhVl99Obry/LeDg7CE7vtF443EeIxO/CVd9L2QXo18GhLtEbBNEpk= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=huaweicloud.com; spf=pass smtp.mailfrom=huaweicloud.com; arc=none smtp.client-ip=14.137.139.154 Authentication-Results: smtp.subspace.kernel.org; dmarc=none (p=none dis=none) header.from=huaweicloud.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=huaweicloud.com Received: from mail.maildlp.com (unknown [172.18.186.29]) by frasgout12.his.huawei.com (SkyGuard) with ESMTP id 4XHDpG46QJz9v7JV; Mon, 30 Sep 2024 16:32:22 +0800 (CST) Received: from mail02.huawei.com (unknown [7.182.16.27]) by mail.maildlp.com (Postfix) with ESMTP id D7AF8140519; Mon, 30 Sep 2024 16:57:56 +0800 (CST) Received: from [10.81.211.60] (unknown [10.81.211.60]) by APP2 (Coremail) with SMTP id GxC2BwCXRscEaPpmNIbwAQ--.5120S2; Mon, 30 Sep 2024 09:57:56 +0100 (CET) Message-ID: <8d20cf79-9fa5-4ced-aa91-232ccd545b59@huaweicloud.com> Date: Mon, 30 Sep 2024 10:57:37 +0200 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH 1/2] compiler.h: Introduce ptr_eq() to preserve address dependency To: Alan Huang , Mathieu Desnoyers Cc: Alan Stern , Linus Torvalds , LKML , Greg Kroah-Hartman , Sebastian Andrzej Siewior , "Paul E. McKenney" , Will Deacon , Peter Zijlstra , Boqun Feng , John Stultz , Neeraj upadhyay , Frederic Weisbecker , Joel Fernandes , Josh Triplett , "Uladzislau Rezki (Sony)" , Steven Rostedt , Lai Jiangshan , Zqiang , Ingo Molnar , Waiman Long , Mark Rutland , Thomas Gleixner , Vlastimil Babka , maged.michael@gmail.com, Mateusz Guzik , Gary Guo , RCU , linux-mm@kvack.org, lkmm@lists.linux.dev References: <20240928135128.991110-1-mathieu.desnoyers@efficios.com> <20240928135128.991110-2-mathieu.desnoyers@efficios.com> <02c63e79-ec8c-4d6a-9fcf-75f0e67ea242@rowland.harvard.edu> <2091628c-2d96-4492-99d9-0f6a61b08d1d@efficios.com> From: Jonas Oberhauser In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 8bit X-CM-TRANSID:GxC2BwCXRscEaPpmNIbwAQ--.5120S2 X-Coremail-Antispam: 1UD129KBjvJXoWxZFyrGr4xCFykJF4Duw45KFg_yoW5AF4DpF W7Ka17KF4kJF1akr90y348uFy5trn7tFyYv3Z5tr1xCws0gF1fZr43tFyYkasxCwn7t34j vr1Yv3sIvasxAaDanT9S1TB71UUUUU7qnTZGkaVYY2UrUUUUjbIjqfuFe4nvWSU5nxnvy2 9KBjDU0xBIdaVrnRJUUUvjb4IE77IF4wAFF20E14v26rWj6s0DM7CY07I20VC2zVCF04k2 6cxKx2IYs7xG6rWj6s0DM7CIcVAFz4kK6r1j6r18M28lY4IEw2IIxxk0rwA2F7IY1VAKz4 vEj48ve4kI8wA2z4x0Y4vE2Ix0cI8IcVAFwI0_Jr0_JF4l84ACjcxK6xIIjxv20xvEc7Cj xVAFwI0_Gr0_Cr1l84ACjcxK6I8E87Iv67AKxVW8JVWxJwA2z4x0Y4vEx4A2jsIEc7CjxV AFwI0_Gr0_Gr1UM2AIxVAIcxkEcVAq07x20xvEncxIr21l5I8CrVACY4xI64kE6c02F40E x7xfMcIj6xIIjxv20xvE14v26r1j6r18McIj6I8E87Iv67AKxVWUJVW8JwAm72CE4IkC6x 0Yz7v_Jr0_Gr1lF7xvr2IY64vIr41lFIxGxcIEc7CjxVA2Y2ka0xkIwI1lc7CjxVAaw2AF wI0_GFv_Wryl42xK82IYc2Ij64vIr41l4I8I3I0E4IkC6x0Yz7v_Jr0_Gr1lx2IqxVAqx4 xG67AKxVWUJVWUGwC20s026x8GjcxK67AKxVWUGVWUWwC2zVAF1VAY17CE14v26r4a6rW5 MIIYrxkI7VAKI48JMIIF0xvE2Ix0cI8IcVAFwI0_Jr0_JF4lIxAIcVC0I7IYx2IY6xkF7I 0E14v26r4j6F4UMIIF0xvE42xK8VAvwI8IcIk0rVWUJVWUCwCI42IY6I8E87Iv67AKxVWU JVW8JwCI42IY6I8E87Iv6xkF7I0E14v26r4j6r4UJbIYCTnIWIevJa73UjIFyTuYvjxUIF 4iUUUUU X-CM-SenderInfo: 5mrqt2oorev25kdx2v3u6k3tpzhluzxrxghudrp/ Am 9/29/2024 um 12:26 AM schrieb Alan Huang: > 2024年9月28日 23:55，Mathieu Desnoyers wrote： >> >> On 2024-09-28 17:49, Alan Stern wrote: >>> On Sat, Sep 28, 2024 at 11:32:18AM -0400, Mathieu Desnoyers wrote: >>>> On 2024-09-28 16:49, Alan Stern wrote: >>>>> On Sat, Sep 28, 2024 at 09:51:27AM -0400, Mathieu Desnoyers wrote: >>>>>> equality, which does not preserve address dependencies and allows the >>>>>> following misordering speculations: >>>>>> >>>>>> - If @b is a constant, the compiler can issue the loads which depend >>>>>> on @a before loading @a. >>>>>> - If @b is a register populated by a prior load, weakly-ordered >>>>>> CPUs can speculate loads which depend on @a before loading @a. >>>>> >>>>> It shouldn't matter whether @a and @b are constants, registers, or >>>>> anything else. All that matters is that the compiler uses the wrong >>>>> one, which allows weakly ordered CPUs to speculate loads you wouldn't >>>>> expect it to, based on the source code alone. >>>> >>>> I only partially agree here. >>>> >>>> On weakly-ordered architectures, indeed we don't care whether the >>>> issue is caused by the compiler reordering the code (constant) >>>> or the CPU speculating the load (registers). >>>> >>>> However, on strongly-ordered architectures, AFAIU, only the constant >>>> case is problematic (compiler reordering the dependent load), because >>> I thought you were trying to prevent the compiler from using one pointer >>> instead of the other, not trying to prevent it from reordering anything. >>> Isn't this the point the documentation wants to get across when it says >>> that comparing pointers can be dangerous? >> >> The motivation for introducing ptr_eq() is indeed because the >> compiler barrier is not sufficient to prevent the compiler from >> using one pointer instead of the other. > > barrier_data(&b) prevents that. I don't think one barrier_data can garantuee preventing this, because right after doing the comparison, the compiler still could do b=a. In that case you would be guaranteed to use the value in b, but that value is not the value loaded into b originally but rather the value loaded into a, and hence your address dependency goes to the wrong load still. However, doing barrier_data(&b); if (a == b) { barrier(); foo(*b); } might maybe prevent it, because after the address of b is escaped, the compiler might no longer be allowed to just do b=a;, but I'm not sure if that is completely correct, since the compiler knows b==a and no other thread can be concurrently modifying a or b. Therefore, given that the compiler knows the hardware, it might know that assigning b=a would not cause any race-related issues even if another thread was reading b concurrently. Finally, it may be only a combination of barrier_data and making b volatile could be guaranteed to solve the issue, but the code will be very obscure compared to using ptr_eq. jonas