From nobody Wed Sep 17 15:52:31 2025 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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 2E55FC46467 for ; Sun, 18 Dec 2022 19:13:40 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230498AbiLRTNa (ORCPT ); Sun, 18 Dec 2022 14:13:30 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:44288 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230427AbiLRTNW (ORCPT ); Sun, 18 Dec 2022 14:13:22 -0500 Received: from mail-qt1-x836.google.com (mail-qt1-x836.google.com [IPv6:2607:f8b0:4864:20::836]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F38BFB870 for ; Sun, 18 Dec 2022 11:13:20 -0800 (PST) Received: by mail-qt1-x836.google.com with SMTP id cg5so6759351qtb.12 for ; Sun, 18 Dec 2022 11:13:20 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:from:to:cc:subject:date :message-id:reply-to; bh=vj6H6w9xKIP6KzPO24qnEpNiKICnr312xMbSiquwZ1w=; b=pt4vXBs6yibcCzpDPgkrFJCClB6mn1GvJAD4rIJtqm5ruo1W3/yInjv/vqqLoHGWV7 AF+fvogP7zG+31uqmGJp3tTlhfgpy9pDMUfo5H4aip1QWDMpt9fDgZNiuTaRExct1+WF hxgnAhD75gKG4kh/Il0+8ELUmXdFXEhTwAHq0= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; h=content-transfer-encoding:mime-version:references:in-reply-to :message-id:date:subject:cc:to:from:x-gm-message-state:from:to:cc :subject:date:message-id:reply-to; bh=vj6H6w9xKIP6KzPO24qnEpNiKICnr312xMbSiquwZ1w=; b=RXeCgp4dtK6gl/Eboc+NgOgVAMHrQqTnKwMbPeCMYMNYLBO1EvBdGVmPyPHCoegpyJ Gjh06HfqBPcMnTcvfbV5A9Qsyu5OsyNDuLt2LIk9qFt9AnhryHn0FOyAbKsY/HTPhTGl 11YR4DeUo5X2QOX2L0MLWCK9kfH2dGHNxIFaGme/2mLVHaD8V09FrzdSHJF2Hsf51xZ9 3HK16Zy+6qR/+/aZcRWJ56+zHs/Eab+Ow8dZE5HEgiVD4DGmGRl3+8SzaRDyuyLmdyse YKmxPwpWQZqssWTtpOiboHd5CwrpquwL9ItX5JEb22+L//WGi5nJ+695oDQpMvv9PjRt rflg== X-Gm-Message-State: ANoB5plcOCEKgczMOR5ij9/7LJ+hByWqHOgUlOLoRRC/smWl6VR1KIYl mWHlMoxXjRO1tyTAAebRo+UWVvVQ6AXsYveKkfU= X-Google-Smtp-Source: AA0mqf42snSRGMzcQvB7R3GsQPL2CIth1zNWtzZ5ziKLDE9SS8vmD/GNAmyZghshkyxYgXjhroqn6w== X-Received: by 2002:ac8:5292:0:b0:3a7:f183:7f66 with SMTP id s18-20020ac85292000000b003a7f1837f66mr54074448qtn.22.1671390799594; Sun, 18 Dec 2022 11:13:19 -0800 (PST) Received: from joelboxx.c.googlers.com.com (48.230.85.34.bc.googleusercontent.com. [34.85.230.48]) by smtp.gmail.com with ESMTPSA id cq8-20020a05622a424800b003a591194221sm4952864qtb.7.2022.12.18.11.13.18 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 18 Dec 2022 11:13:18 -0800 (PST) From: "Joel Fernandes (Google)" To: linux-kernel@vger.kernel.org Cc: "Joel Fernandes (Google)" , Josh Triplett , Lai Jiangshan , Mathieu Desnoyers , "Paul E. McKenney" , rcu@vger.kernel.org, Steven Rostedt Subject: [RFC 2/2] srcu: Remove memory barrier "E" as it is not required Date: Sun, 18 Dec 2022 19:13:09 +0000 Message-Id: <20221218191310.130904-3-joel@joelfernandes.org> X-Mailer: git-send-email 2.39.0.314.g84b9a713c41-goog In-Reply-To: <20221218191310.130904-1-joel@joelfernandes.org> References: <20221218191310.130904-1-joel@joelfernandes.org> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" During a flip, we have a full memory barrier before idx is incremented. The effect of this seems to be to guarantee that, if a READER sees srcu_idx updates (srcu_flip), then prior scans would not see its updates to counters= on that index. That does not matter because of the following reason: If a prior scan did s= ee counter updates on the new index, that means the prior scan would would wait for the reader when it probably did not need to. And if the prior scan did see both lock and unlock count updates on that in= dex, that reader is essentially done, so it is OK to end the grace period. For this reason, remove the full memory barrier before incrementing srcu_idx. 6 hours of testing shows all SRCU-* scenarios pass with this. Signed-off-by: Joel Fernandes (Google) --- kernel/rcu/srcutree.c | 8 -------- 1 file changed, 8 deletions(-) diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index d6a4c2439ca6..2d2e6d304a43 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -982,14 +982,6 @@ static bool try_check_zero(struct srcu_struct *ssp, in= t idx, int trycount) */ static void srcu_flip(struct srcu_struct *ssp) { - /* - * Ensure that if a given reader sees the new value of ->srcu_idx, this - * updater's earlier scans cannot have seen that reader's increments - * (which is OK, because this grace period need not wait on that - * reader). - */ - smp_mb(); /* E */ /* Pairs with B and C. */ - WRITE_ONCE(ssp->srcu_idx, ssp->srcu_idx + 1); =20 /* --=20 2.39.0.314.g84b9a713c41-goog