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 X-Spam-Level: X-Spam-Status: No, score=-9.8 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 565C1C54FD0 for ; Mon, 20 Apr 2020 15:39:06 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 35013206F6 for ; Mon, 20 Apr 2020 15:39:06 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="QXhjMscJ" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728514AbgDTPjF (ORCPT ); Mon, 20 Apr 2020 11:39:05 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:60312 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727837AbgDTPjC (ORCPT ); Mon, 20 Apr 2020 11:39:02 -0400 Received: from mail-qt1-x843.google.com (mail-qt1-x843.google.com [IPv6:2607:f8b0:4864:20::843]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id BF196C061A0C for ; Mon, 20 Apr 2020 08:39:01 -0700 (PDT) Received: by mail-qt1-x843.google.com with SMTP id c16so8847238qtv.1 for ; Mon, 20 Apr 2020 08:39:01 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=lcc0Wlav0FsTVJwVQ2AzBgPdfSD4p+1T7p29iEMrsoA=; b=QXhjMscJLEN1Xb7/cRhdNbtsc/FTLmDNHUqXblfXO+STa/oRDtChQgBRItNEcP3eNn 1Wh24G9Wu1Nl6cdTVGWxZnJTp27TCv2bkujDGXmhMsvRIin/WNCgvNzVlLdT1JCDYlS6 hhMgZlC6UQvY5QRZakDyzZWOaYP9x1frn2V1g= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:from:to:cc:subject:date:message-id:in-reply-to :references:mime-version:content-transfer-encoding; bh=lcc0Wlav0FsTVJwVQ2AzBgPdfSD4p+1T7p29iEMrsoA=; b=ECHHT4dznRMsD9v3Tm4x/IGgZoo6VFX2W5A821K5/yPW9Nab8qfC5ESn5qRMkVxW/s 2AMHjf+a/hLub4wx2ALKlfRJIpWzdaBXFJK1tWo4Ujp1hZXUtkeqeI0D0Ml+5pqDzNNF S9aor6k1bDadkAVP8ZksMPU9mEt+CZ9gkozRWReOf87MBTNnwVfXlTZINq+NXEYa/U7L 8BTZnoxuRu/nhKATq3JsU8lSeptzo9xQRIkJMD4vEX1LXKRt9VANWVmd41EBjBmysPh8 v81FVXEtPulTeFpXSpGA5SV4bV38BKx1maqYKPVMiMWTAODRdYvisEiDYQ+eSIWwyBgK ICrA== X-Gm-Message-State: AGi0PuZD/IJOqETGfk/iyCekQ5iuEVAxV0Jx0gwjLYAuWvq/MaWODB1t 2CUBY8V3jEvCc1oqYXNlb72gtA== X-Google-Smtp-Source: APiQypIPDukJ93IS4iW0yXmsvSzB3DJRdlglu/CEk9JLy+UI3nLbWM+Ln2cWb3eT1ITT86TzrWgiCA== X-Received: by 2002:ac8:5256:: with SMTP id y22mr15799428qtn.321.1587397140926; Mon, 20 Apr 2020 08:39:00 -0700 (PDT) Received: from joelaf.cam.corp.google.com ([2620:15c:6:12:9c46:e0da:efbf:69cc]) by smtp.gmail.com with ESMTPSA id v16sm765453qkf.80.2020.04.20.08.38.59 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Mon, 20 Apr 2020 08:39:00 -0700 (PDT) From: "Joel Fernandes (Google)" To: linux-kernel@vger.kernel.org Cc: Sebastian Andrzej Siewior , Joel Fernandes , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , "Paul E. McKenney" , rcu@vger.kernel.org, Steven Rostedt , urezki@gmail.com Subject: [PATCH rcu/dev -fixes 3/4] rcu/tree: Avoid using xchg() in kfree_call_rcu_add_ptr_to_bulk() Date: Mon, 20 Apr 2020 11:38:36 -0400 Message-Id: <20200420153837.194532-4-joel@joelfernandes.org> X-Mailer: git-send-email 2.26.1.301.g55bc3eb7cb9-goog In-Reply-To: <20200420153837.194532-1-joel@joelfernandes.org> References: <20200420153837.194532-1-joel@joelfernandes.org> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Sender: rcu-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: rcu@vger.kernel.org From: Sebastian Andrzej Siewior There is no need to use xchg(), the access is serialized by krcp->lock. The xchg() function adds some atomic barriers which remain hidden in x86's disassembly but are visible on ARM for instance. Replace xchg() with a load + store. Acked-by: Joel Fernandes (Google) Signed-off-by: Sebastian Andrzej Siewior --- kernel/rcu/tree.c | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c index cd61649e1b001..f6eb3aee0935e 100644 --- a/kernel/rcu/tree.c +++ b/kernel/rcu/tree.c @@ -3088,7 +3088,8 @@ kfree_call_rcu_add_ptr_to_bulk(struct kfree_rcu_cpu *krcp, /* Check if a new block is required. */ if (!krcp->bhead || krcp->bhead->nr_records == KFREE_BULK_MAX_ENTR) { - bnode = xchg(&krcp->bcached, NULL); + bnode = krcp->bcached; + krcp->bcached = NULL; if (!bnode) { WARN_ON_ONCE(sizeof(struct kfree_rcu_bulk_data) > PAGE_SIZE); -- 2.26.1.301.g55bc3eb7cb9-goog