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 BC84FCF34A9 for ; Thu, 3 Oct 2024 13:32:58 +0000 (UTC) Received: by kanga.kvack.org (Postfix) id 3AC0C6B00AF; Thu, 3 Oct 2024 09:32:58 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 35BDA6B0145; Thu, 3 Oct 2024 09:32:58 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id 1FD166B0144; Thu, 3 Oct 2024 09:32:58 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0012.hostedemail.com [216.40.44.12]) by kanga.kvack.org (Postfix) with ESMTP id F3CED6B048F for ; Thu, 3 Oct 2024 09:32:57 -0400 (EDT) Received: from smtpin10.hostedemail.com (a10.router.float.18 [10.200.18.1]) by unirelay07.hostedemail.com (Postfix) with ESMTP id 8C0BC161613 for ; Thu, 3 Oct 2024 13:32:57 +0000 (UTC) X-FDA: 82632381594.10.3EFEE88 Received: from smtpout.efficios.com (smtpout.efficios.com [167.114.26.122]) by imf04.hostedemail.com (Postfix) with ESMTP id D482140006 for ; Thu, 3 Oct 2024 13:32:55 +0000 (UTC) Authentication-Results: imf04.hostedemail.com; dkim=pass header.d=efficios.com header.s=smtpout1 header.b="X1GK8v/O"; spf=pass (imf04.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=1727962246; 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=EYQZwuOM1d6hBgeud4aLmkm+ljW2O/hhg6D7Xh9gZxY=; b=VReoWzssfEVy8AGgcfhJsq3cQQcG3UN3w0tCCoqNS0VIYEJoc+Vv5E+lZWNhgvxXXAGm0X dUtUHo5lhgntyky7+wZwWwN/CZ08GPm2syCSUMBDQwABAvrHEptuZ+HnALzh78ognM9uiV baTtpW/iaTnPEiZroSl1SvdYwmwtkbM= ARC-Seal: i=1; s=arc-20220608; d=hostedemail.com; t=1727962246; a=rsa-sha256; cv=none; b=qc8749eHk/kWDxdiqElvS3EbJP3L1aVDe/WV0/YVFivmIuAOyga/ifz/o6uN+X4+Do47hw 12Gi/zhSezP54PTp/RH2znwT+W4Pi7nxB6FaJQ3GOiDzv9GoON6Y9NOP25wjKquknvY4xQ HG0N0/oaZWYWCuG9HbU4Fe/w3zL8I8E= ARC-Authentication-Results: i=1; imf04.hostedemail.com; dkim=pass header.d=efficios.com header.s=smtpout1 header.b="X1GK8v/O"; spf=pass (imf04.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 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=efficios.com; s=smtpout1; t=1727962375; bh=YTSZ1gR4pl10kKX9+QjBQhs9v6DfHc5Ds/ZvHb/XL/g=; h=Date:Subject:To:Cc:References:From:In-Reply-To:From; b=X1GK8v/OZtESLEHkimlfFxoawsowDAJytBMD2o7/U0FjMAMyY0u+RROxIqV1qNfcO 3VbzfSyj5e74S4TZIP+m3k83XPJdJNWh/ZRB8KvKn7KUAb6g7wvOlIjP+2ynmX6EWw I1hoTW40T0gJ8+mI7oLSWsRRU+KbCt2Lr3PndOE1bIwNRv37f57wCKuVGJ6CaybKuh 4h7vJ/CxdQBINIqr6rnGhWb9eGjU+IV3tL5x2CgUP73uxbOv13P7MtRIjp6p6RAfbj QwGn8sBl9bSG4nUAImNPfpOK1upeqFdU8V3+q3bwlJt0qieL+URAbG8uzQ1lmatJi7 RovViSGiBjP1A== Received: from [172.16.0.134] (96-127-217-162.qc.cable.ebox.net [96.127.217.162]) by smtpout.efficios.com (Postfix) with ESMTPSA id 4XKCKf3WScz5Zd; Thu, 3 Oct 2024 09:32:54 -0400 (EDT) Message-ID: Date: Thu, 3 Oct 2024 09:30:53 -0400 MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [RFC PATCH 3/4] hp: Implement Hazard Pointers To: Boqun Feng Cc: Linus Torvalds , Andrew Morton , Peter Zijlstra , linux-kernel@vger.kernel.org, Nicholas Piggin , Michael Ellerman , Greg Kroah-Hartman , Sebastian Andrzej Siewior , "Paul E. McKenney" , Will Deacon , Alan Stern , John Stultz , Neeraj Upadhyay , Frederic Weisbecker , Joel Fernandes , Josh Triplett , Uladzislau Rezki , Steven Rostedt , Lai Jiangshan , Zqiang , Ingo Molnar , Waiman Long , Mark Rutland , Thomas Gleixner , Vlastimil Babka , maged.michael@gmail.com, Mateusz Guzik , Jonas Oberhauser , rcu@vger.kernel.org, linux-mm@kvack.org, lkmm@lists.linux.dev References: <20241002010205.1341915-1-mathieu.desnoyers@efficios.com> <20241002010205.1341915-4-mathieu.desnoyers@efficios.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-Rspam-User: X-Rspamd-Server: rspam04 X-Rspamd-Queue-Id: D482140006 X-Stat-Signature: xkr9z89hqmygqs7zinf1wq3tuz75mkbt X-HE-Tag: 1727962375-927113 X-HE-Meta: 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 qkZAi2bn lwpPK50URMSwrblZmNuXIOBbJg9C3Mw7r2xJUWsTfylq+Qo1TpoEtyZ0LAGdxOIRXEjLMwfSSzKoeZFszVOnE+bbcKafsJkHjGDLlm21N2uHb5XflGR2/Z2c5BiBzzK5tSCQbTwYx4Zgki+XaBtuFy2WVGq8KT9jDcyHaGB4Nc+kDf8WAlF9XW3gJNx5uYR8ffwHuLj22eRNujWcxLIlMRnYtDOXJNipvgtHgvQcSqILgllYyz7dzNtaXuYTyVrSc/Nsz/RGECRgEk3ZGoPZYyiVCEDEcHZ0zfCd3Cah5z7BjKrd7IciMgGf3KqDWxvvE49VYGvQqpPwiU1WdAFkQ+HdzEaF+asEE/vyw58WJfPMIVINDxAGMNPKZ8HQh9HZmgiREZVT5sZtelGSTEYjQldU2ZB7afBdWtOcr4o9Gzffw40c= 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 2024-10-03 02:24, Boqun Feng wrote: > On Tue, Oct 01, 2024 at 09:02:04PM -0400, Mathieu Desnoyers wrote: [...] >> +/* >> + * hp_allocate: Allocate a hazard pointer. >> + * >> + * Allocate a hazard pointer slot for @addr. The object existence should >> + * be guaranteed by the caller. >> + * >> + * Returns a hazard pointer context. >> + */ >> +static inline >> +struct hp_ctx hp_allocate(struct hp_slot __percpu *percpu_slots, void *addr) >> +{ >> + struct hp_slot *slot; >> + struct hp_ctx ctx; >> + >> + if (!addr) >> + goto fail; >> + slot = this_cpu_ptr(percpu_slots); > > Are you assuming this is called with preemption disabled? Otherwise, > there could two threads picking up the same hazard pointer slot on one > CPU, Indeed, this minimalist implementation only covers the preempt-off use-case, where there is a single use of HP per CPU at any given time (e.g. for the lazy mm use-case). It expects to be called from preempt-off context. I will update the comment accordingly. > >> + /* >> + * A single hazard pointer slot per CPU is available currently. >> + * Other hazard pointer domains can eventually have a different >> + * configuration. >> + */ >> + if (READ_ONCE(slot->addr)) >> + goto fail; > > .. and they could both read an empty slot, and both think they > successfully protect the objects, which could be different objects. > > Or am I missing something subtle here? You are correct, I should document this. > >> + WRITE_ONCE(slot->addr, addr); /* Store B */ >> + ctx.slot = slot; >> + ctx.addr = addr; >> + return ctx; >> + >> +fail: >> + ctx.slot = NULL; >> + ctx.addr = NULL; >> + return ctx; >> +} >> + >> +/* >> + * hp_dereference_allocate: Dereference and allocate a hazard pointer. >> + * >> + * Returns a hazard pointer context. >> + */ >> +static inline >> +struct hp_ctx hp_dereference_allocate(struct hp_slot __percpu *percpu_slots, void * const * addr_p) >> +{ >> + struct hp_slot *slot; >> + void *addr, *addr2; >> + struct hp_ctx ctx; >> + >> + addr = READ_ONCE(*addr_p); >> +retry: >> + ctx = hp_allocate(percpu_slots, addr); >> + if (!hp_ctx_addr(ctx)) >> + goto fail; >> + /* Memory ordering: Store B before Load A. */ >> + smp_mb(); >> + /* >> + * Use RCU dereference without lockdep checks, because >> + * lockdep is not aware of HP guarantees. >> + */ >> + addr2 = rcu_access_pointer(*addr_p); /* Load A */ > > Why rcu_access_pointer() instead of READ_ONCE()? Because you want to > mark the head of address dependency? Yes, the intent here is to mark the address dependency and provide a publication guarantee similar to RCU pairing rcu_assign_pointer and rcu_dereference. Do you see any reason why READ_ONCE() would suffice here ? Thanks, Mathieu > > Regards, > Boqun > >> + /* >> + * If @addr_p content has changed since the first load, >> + * clear the hazard pointer and try again. >> + */ >> + if (!ptr_eq(addr2, addr)) { >> + WRITE_ONCE(slot->addr, NULL); >> + if (!addr2) >> + goto fail; >> + addr = addr2; >> + goto retry; >> + } >> + ctx.slot = slot; >> + ctx.addr = addr2; >> + return ctx; >> + >> +fail: >> + ctx.slot = NULL; >> + ctx.addr = NULL; >> + return ctx; >> +} >> + > [...] -- Mathieu Desnoyers EfficiOS Inc. https://www.efficios.com