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 kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.lore.kernel.org (Postfix) with ESMTP id 19DFDC369D5 for ; Wed, 25 Sep 2024 12:00:01 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 6395D6B0098; Wed, 25 Sep 2024 08:00:00 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 5E9236B009A; Wed, 25 Sep 2024 08:00:00 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 4D81A6B009B; Wed, 25 Sep 2024 08:00:00 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0013.hostedemail.com [216.40.44.13]) by kanga.kvack.org (Postfix) with ESMTP id 304B06B0098 for ; Wed, 25 Sep 2024 08:00:00 -0400 (EDT) Received: from smtpin03.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay08.hostedemail.com (Postfix) with ESMTP id C12B7140248 for ; Wed, 25 Sep 2024 11:59:59 +0000 (UTC) X-FDA: 82603116918.03.37DFE9A Received: from smtpout.efficios.com (smtpout.efficios.com [167.114.26.122]) by imf27.hostedemail.com (Postfix) with ESMTP id 13E6C40014 for ; Wed, 25 Sep 2024 11:59:57 +0000 (UTC) Authentication-Results: imf27.hostedemail.com; dkim=pass header.d=efficios.com header.s=smtpout1 header.b=rlH3JZhc; spf=pass (imf27.hostedemail.com: domain of mathieu.desnoyers@efficios.com designates 167.114.26.122 as permitted sender) smtp.mailfrom=mathieu.desnoyers@efficios.com; dmarc=pass (policy=none) header.from=efficios.com ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1727265438; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=ri+IIjkYigOFqyAfLEPfYDR4yQYfJZ61wCUMkHVRrbY=; b=eGZAUGX9PamTrYRAagrB/M0jRyZn+w6WFzXbnKk3VDANvvuvZmtCc6nwJbo0tJUZxdT8cm bnbg5nr03LXh3kohjAQNSdNItSPiUqvxnx2Hj4OlQhpgIOckRPaJza/VfoRmCTlz9VN2Ot jbHGm7xpzASHwadWOZvkRSJ1G+5eTZk= ARC-Authentication-Results: i=1; imf27.hostedemail.com; dkim=pass header.d=efficios.com header.s=smtpout1 header.b=rlH3JZhc; spf=pass (imf27.hostedemail.com: domain of mathieu.desnoyers@efficios.com designates 167.114.26.122 as permitted sender) smtp.mailfrom=mathieu.desnoyers@efficios.com; dmarc=pass (policy=none) header.from=efficios.com ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1727265438; a=rsa-sha256; cv=none; b=QWc/0EdgjrGtFwVX0X/APvMDlU7Lk6LGaKxcW3UCYUuLz9pltXn7C9Tq9u2Y5ndRO1uztc ctwhS1w+88QdN4P7k8ciL8nr9OtKjM2vEZRWUKh3DDSR3BmQfn8Vp7tHsCYzpPqW8c/wS/ GkZYGLHDrvLfWWzQbZcrjP8reM0yRgA= DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=efficios.com; s=smtpout1; t=1727265597; bh=W7bIXX4Xs7iiomVoHGKvUg62JxTlQSuNM0jVYBKFCDo=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=rlH3JZhcD0hlsfIxRSykteKfS6UP3flhZBIz8ovXjkbvQLJd+/79GkBaccpm5Fj2K FNuQRXMZWCvYk/pch6SL1JjRWEL41u8xgsaFaTUNEfUVx3VFzY/WHMs9uNHnCn2IOC mdmu7H83iP/OQYf+vQgD+VFtqo0GnaloIgd6RkjSCu07mPbU3mMsTNFCop69HDAGIf lK3iWa4juqjRlwR5+5VI2g+gk6Y3eAO8kZjP8rA68im2ODFtxgSSnGEpbf1c+CNY5Q FRAzb6JGfxHV5zpjz2I71YvCyrqt8vZxufganosyz82TgmUFiHRKqnNcuMAZn/d0nR hYqYFGRm/Qlog== Received: from [192.168.126.112] (unknown [147.75.204.251]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4XDFdt4nT3z1M9F; Wed, 25 Sep 2024 07:59:46 -0400 (EDT) Message-ID: <4167e6f5-4ff9-4aaa-915e-c1e692ac785a@efficios.com> Date: Wed, 25 Sep 2024 13:59:06 +0200 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 1/4] hazptr: Add initial implementation of hazard pointers To: Boqun Feng , Jonas Oberhauser Cc: linux-kernel@vger.kernel.org, rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev, "Paul E. McKenney" , Frederic Weisbecker , Neeraj Upadhyay , Joel Fernandes , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Lai Jiangshan , Zqiang , Peter Zijlstra , Ingo Molnar , Will Deacon , Waiman Long , Mark Rutland , Thomas Gleixner , Kent Overstreet , Linus Torvalds , Vlastimil Babka , maged.michael@gmail.com, Neeraj Upadhyay References: <20240917143402.930114-1-boqun.feng@gmail.com> <20240917143402.930114-2-boqun.feng@gmail.com> <55975a55-302f-4c45-bfcc-192a8a1242e9@huaweicloud.com> From: Mathieu Desnoyers Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Rspamd-Server: rspam06 X-Rspamd-Queue-Id: 13E6C40014 X-Stat-Signature: 8ptffpnwp63rcp59e64n4yzt5tez5ghq X-Rspam-User: X-HE-Tag: 1727265597-881295 X-HE-Meta: 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 FP7xKmu5 3VTpDw/zdnI9bSu9MEvBZ1iuurkCq5vbZf1FTroLlcoOql5edi55Eu41loGYAdIpIgQ1p6SJ3/7IyRBttYkiVxYofEBX2WYtp8MkDh9CnYm92SUhqszSpgKfoXQ2HBNa7YsHChZlzkoxedKlxukuWVXwfjvweiTtlaBZO00jwJiMPEtptfDLNzJ8POqb9r2FqPIQ6QNDsdcdlNBwMEsA4zg46U0WS84dOBWjazc2KTLDC1eMD+TJZT6mtLMYtm6wjw9i4bRX2s1TsBbVSuYYg7JLDhr0yDS0JtANd+gWTmmv+R29x6hu2JL7/9SVAfvMS7otPQa3U5GltzW2grKCgeB4Mgr8yQYTieh14n/d1E89iBgqQobpLtYQR43IR00UkflpHtyB0M+l/ukporrYMhK7ae+xCSpAbOxbexUYvCV4qJoM= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: On 2024-09-25 12:45, Boqun Feng wrote: > On Wed, Sep 25, 2024 at 12:11:52PM +0200, Jonas Oberhauser wrote: >> >> >> Am 9/25/2024 um 12:02 PM schrieb Boqun Feng: >>> Hi Jonas, >>> >>> Of >>> course, if we are really worried about compilers being too "smart" >> >> Ah, I see you know me better and better... >> >>> we can always do the comparison in asm code, then compilers don't know >>> anything of the equality between 'ptr' and 'head - head_offset'. >> Yes, but then a simple compiler barrier between the comparison and returning >> ptr would also do the trick, right? And maybe easier on the eyes. >> > > The thing about putting a compiler barrier is that it will prevent all > compiler reorderings, and some of the reordering may contribute to > better codegen. (I know in this case, we have a smp_mb(), but still > compilers can move unrelated code upto the second load for optimization > purpose). Asm comparison is cheaper in this way. But TBH, compilers > should provide a way to compare pointer values without using the result > for pointer equality proof, if "convert to unsigned long" doesn't work, > some other ways should work. > Based on Documentation/RCU/rcu_dereference.rst : - Be very careful about comparing pointers obtained from rcu_dereference() against non-NULL values. As Linus Torvalds explained, if the two pointers are equal, the compiler could substitute the pointer you are comparing against for the pointer obtained from rcu_dereference(). For example:: p = rcu_dereference(gp); if (p == &default_struct) do_default(p->a); Because the compiler now knows that the value of "p" is exactly the address of the variable "default_struct", it is free to transform this code into the following:: p = rcu_dereference(gp); if (p == &default_struct) do_default(default_struct.a); On ARM and Power hardware, the load from "default_struct.a" can now be speculated, such that it might happen before the rcu_dereference(). This could result in bugs due to misordering. So I am not only concerned about compiler proofs here, as it appears that the speculation done by the CPU can also cause issues on some architectures. Thanks, Mathieu > Regards, > Boqun > >> >> Have fun, >> jonas >> -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com