From patchwork Thu Nov 30 18:28:33 2017 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Will Deacon X-Patchwork-Id: 10085531 Return-Path: Received: from mail.wl.linuxfoundation.org (pdx-wl-mail.web.codeaurora.org [172.30.200.125]) by pdx-korg-patchwork.web.codeaurora.org (Postfix) with ESMTP id 939A360234 for ; Thu, 30 Nov 2017 18:28:58 +0000 (UTC) Received: from mail.wl.linuxfoundation.org (localhost [127.0.0.1]) by mail.wl.linuxfoundation.org (Postfix) with ESMTP id 8B6162A263 for ; Thu, 30 Nov 2017 18:28:58 +0000 (UTC) Received: by mail.wl.linuxfoundation.org (Postfix, from userid 486) id 803002A272; Thu, 30 Nov 2017 18:28:58 +0000 (UTC) X-Spam-Checker-Version: SpamAssassin 3.3.1 (2010-03-16) on pdx-wl-mail.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-4.2 required=2.0 tests=BAYES_00,DKIM_SIGNED, DKIM_VALID,RCVD_IN_DNSWL_MED autolearn=ham version=3.3.1 Received: from bombadil.infradead.org (bombadil.infradead.org [65.50.211.133]) (using TLSv1.2 with cipher AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.wl.linuxfoundation.org (Postfix) with ESMTPS id E6FE42A274 for ; Thu, 30 Nov 2017 18:28:57 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20170209; h=Sender: Content-Transfer-Encoding:Content-Type:MIME-Version:Cc:List-Subscribe: List-Help:List-Post:List-Archive:List-Unsubscribe:List-Id:Message-Id:Date: Subject:To:From:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:In-Reply-To: References:List-Owner; bh=ZbrEdsFm1CyZHkiNQeqcUJMI/y+stmuGjP2K8SUQbpw=; b=PDG FPf2nxFZpORW5GiTcpc4Y7O6KeWd8w9VSlyskWeBrZRLlksHOloANbNaTo9OSh3wj9r+Yy7cIuaXH rcoY2OgYcoKJ/z6TI3DX13976fNAzy6KAiw3pdP1sgrI1Pj16thvoiQTCFsH5FTau22dIAEbsIXWG DRwiWPSXY61SzZSdZBTATX12oZTvKNMLhoIQ2mGAHEcGqmo18HsJbjWJ4II+6BtVF5ZcatoDZhmZz S3xDtB1hTdZCKBkhhm95iJVEf8/UNIHlY3sPyHLIwZ79vsHEokECPonED8WBHgQjtjyJ0jwgFa/QU o98rJDebGmfhitYICkzlfcpJL9bv9kg==; Received: from localhost ([127.0.0.1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.87 #1 (Red Hat Linux)) id 1eKTZx-0001Gf-9H; Thu, 30 Nov 2017 18:28:57 +0000 Received: from usa-sjc-mx-foss1.foss.arm.com ([217.140.101.70] helo=foss.arm.com) by bombadil.infradead.org with esmtp (Exim 4.87 #1 (Red Hat Linux)) id 1eKTZt-0001Eq-QY for linux-arm-kernel@lists.infradead.org; Thu, 30 Nov 2017 18:28:55 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.72.51.249]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id BA61E1435; Thu, 30 Nov 2017 10:28:32 -0800 (PST) Received: from edgewater-inn.cambridge.arm.com (usa-sjc-imap-foss1.foss.arm.com [10.72.51.249]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPA id 89C0E3F236; Thu, 30 Nov 2017 10:28:32 -0800 (PST) Received: by edgewater-inn.cambridge.arm.com (Postfix, from userid 1000) id D7DF61AE3BC8; Thu, 30 Nov 2017 18:28:35 +0000 (GMT) From: Will Deacon To: linux-arm-kernel@lists.infradead.org Subject: [PATCH] arm64: context: Fix comments and remove pointless smp_wmb() Date: Thu, 30 Nov 2017 18:28:33 +0000 Message-Id: <1512066513-16024-1-git-send-email-will.deacon@arm.com> X-Mailer: git-send-email 2.1.4 X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20171130_102853_874103_B57FB64A X-CRM114-Status: UNSURE ( 9.78 ) X-CRM114-Notice: Please train this message. X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.21 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: catalin.marinas@arm.com, Will Deacon , James Morse MIME-Version: 1.0 Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+patchwork-linux-arm=patchwork.kernel.org@lists.infradead.org X-Virus-Scanned: ClamAV using ClamSMTP The comments in the ASID allocator incorrectly hint at an MP-style idiom using the asid_generation and the active_asids array. In fact, the synchronisation is achieved using a combination of an xchg operation and a spinlock, so update the comments and remove the pointless smp_wmb(). Cc: James Morse Signed-off-by: Will Deacon --- arch/arm64/mm/context.c | 23 ++++++++++++----------- 1 file changed, 12 insertions(+), 11 deletions(-) diff --git a/arch/arm64/mm/context.c b/arch/arm64/mm/context.c index ab9f5f0fb2c7..98fcecae0827 100644 --- a/arch/arm64/mm/context.c +++ b/arch/arm64/mm/context.c @@ -96,12 +96,6 @@ static void flush_context(unsigned int cpu) set_reserved_asid_bits(); - /* - * Ensure the generation bump is observed before we xchg the - * active_asids. - */ - smp_wmb(); - for_each_possible_cpu(i) { asid = atomic64_xchg_relaxed(&per_cpu(active_asids, i), 0); /* @@ -202,11 +196,18 @@ void check_and_switch_context(struct mm_struct *mm, unsigned int cpu) asid = atomic64_read(&mm->context.id); /* - * The memory ordering here is subtle. We rely on the control - * dependency between the generation read and the update of - * active_asids to ensure that we are synchronised with a - * parallel rollover (i.e. this pairs with the smp_wmb() in - * flush_context). + * The memory ordering here is subtle. + * If our ASID matches the current generation, then we update + * our active_asids entry with a relaxed xchg. Racing with a + * concurrent rollover means that either: + * + * - We get a zero back from the xchg and end up waiting on the + * lock. Taking the lock synchronises with the rollover and so + * we are forced to see the updated generation. + * + * - We get a valid ASID back from the xchg, which means the + * relaxed xchg in flush_context will treat us as reserved + * because atomic RmWs are totally ordered for a given location. */ if (!((asid ^ atomic64_read(&asid_generation)) >> asid_bits) && atomic64_xchg_relaxed(&per_cpu(active_asids, cpu), asid))