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=-12.8 required=3.0 tests=BAYES_00,DKIM_INVALID, DKIM_SIGNED,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH,MAILING_LIST_MULTI, SIGNED_OFF_BY,SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,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 B98BFC433DF for ; Fri, 7 Aug 2020 19:17:03 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 7EB8B214F1 for ; Fri, 7 Aug 2020 19:17:03 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=gmail.com header.i=@gmail.com header.b="UfqzL8BW" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1726344AbgHGTRB (ORCPT ); Fri, 7 Aug 2020 15:17:01 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:33158 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1725893AbgHGTQ6 (ORCPT ); Fri, 7 Aug 2020 15:16:58 -0400 Received: from mail-pf1-x444.google.com (mail-pf1-x444.google.com [IPv6:2607:f8b0:4864:20::444]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id A2B09C061756 for ; Fri, 7 Aug 2020 12:16:58 -0700 (PDT) Received: by mail-pf1-x444.google.com with SMTP id d188so1546137pfd.2 for ; Fri, 07 Aug 2020 12:16:58 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gmail.com; s=20161025; h=sender:from:to:cc:subject:date:message-id:in-reply-to:references :mime-version:content-transfer-encoding; bh=nAulkNxU9Nrr0r2Q+/oP/y0eVROiBvJb3wM5+KKQ5eM=; b=UfqzL8BWBezA23AIgV8lJCXtRB83oKAQHXP40B+Y2CnU/8eLW4EQ9ejYtDvdiiOwkX wPAh+O40EoFMjNWCqsOUF2+EgSRDJJc/yKiqVbZw9quOyfJORy+b50B+akPFnX0HOhB3 DiyIeKi/H2h6tE15XOQCKjLWwxbUJud4qH6DhM08w8eT8SAS7MhF817/+Pw+FmpcNEal H5rfjlUhG9/hN1zcuAVwlkShWvvz8JlViKfMuoRFl+On6EY6qgEmUYNx/pfem7r27wBe ByRK5gJcCGeh9i9K8yPg8qUz5tUIWXHdMo7iOLl1waPAe6AugLjFtlXRW57QHcwrDchK Y4RA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:from:to:cc:subject:date:message-id :in-reply-to:references:mime-version:content-transfer-encoding; bh=nAulkNxU9Nrr0r2Q+/oP/y0eVROiBvJb3wM5+KKQ5eM=; b=i3Q1Jupqgo0inegFmg0+EEHv7C2gcVtZseHBhlCuXbCL+jkW6objr1yQ9jK4VP5YhU 011wAtlfKwUmrgdQxvNsghjARP/nf4qHunrM9f2NnXo//z8dJb69cB6lczLEt4uoeZNb Lyjm7y0dIAK+evO8Nu8nYOTs+QwKarIaW75coBJv7TLERgNJOjjEkFIa2dra1GlpXvIJ St1KZv34ykUlPKMeVZ/HEW0pEi7juM9X+rfQktVx43mLDkoLW6oVLeSUPoWLjhAw+dNE 2t2h9go6vLf3ehVYX7EAlilNlg7yOPZY6FO2gqs3glYQlYYvM3DB5l/WTxSU5eGuEfUW bFFA== X-Gm-Message-State: AOAM531zDfKgJFhMA6XLlOyIL9UN2ZX7ARjiZBdeETLqhxz2SnHR/akq pJjxZvu96LgtaNEhwyi61qM= X-Google-Smtp-Source: ABdhPJxlcMhidUm9kO+7tjlgQ6RAcIOD9PZs5RerRs47ASqhXiA2lHjyeI0ftKa71wrIekEwLEHYzA== X-Received: by 2002:a65:63c8:: with SMTP id n8mr12757511pgv.232.1596827818163; Fri, 07 Aug 2020 12:16:58 -0700 (PDT) Received: from sultan-box.localdomain ([104.200.129.62]) by smtp.gmail.com with ESMTPSA id y4sm13432665pff.44.2020.08.07.12.16.57 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Fri, 07 Aug 2020 12:16:57 -0700 (PDT) From: Sultan Alsawaf X-Google-Original-From: Sultan Alsawaf Cc: Peter Zijlstra , Ingo Molnar , Will Deacon , linux-kernel@vger.kernel.org, Sultan Alsawaf Subject: [PATCH 2/2] locking/rwsem: Don't hog RCU read lock while optimistically spinning Date: Fri, 7 Aug 2020 12:16:36 -0700 Message-Id: <20200807191636.75045-2-sultan@kerneltoast.com> X-Mailer: git-send-email 2.28.0 In-Reply-To: <20200807191636.75045-1-sultan@kerneltoast.com> References: <20200807191636.75045-1-sultan@kerneltoast.com> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit To: unlisted-recipients:; (no To-header on input) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Sultan Alsawaf There's no reason to hold an RCU read lock the entire time while optimistically spinning for a rwsem. This can needlessly lengthen RCU grace periods and slow down synchronize_rcu() when it doesn't brute force the RCU grace period via rcupdate.rcu_expedited=1. Signed-off-by: Sultan Alsawaf --- kernel/locking/rwsem.c | 27 +++++++++++++++++---------- 1 file changed, 17 insertions(+), 10 deletions(-) diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c index f11b9bd3431d..a1e3ceb254d1 100644 --- a/kernel/locking/rwsem.c +++ b/kernel/locking/rwsem.c @@ -723,8 +723,10 @@ rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable) if (state != OWNER_WRITER) return state; - rcu_read_lock(); for (;;) { + bool same_owner; + + rcu_read_lock(); /* * When a waiting writer set the handoff flag, it may spin * on the owner as well. Once that writer acquires the lock, @@ -732,27 +734,32 @@ rwsem_spin_on_owner(struct rw_semaphore *sem, unsigned long nonspinnable) * handoff bit is set. */ new = rwsem_owner_flags(sem, &new_flags); - if ((new != owner) || (new_flags != flags)) { - state = rwsem_owner_state(new, new_flags, nonspinnable); - break; - } /* - * Ensure we emit the owner->on_cpu, dereference _after_ - * checking sem->owner still matches owner, if that fails, + * Ensure sem->owner still matches owner. If that fails, * owner might point to free()d memory, if it still matches, * the rcu_read_lock() ensures the memory stays valid. */ - barrier(); + same_owner = new == owner && new_flags == flags; + if (same_owner && !owner_on_cpu(owner)) + state = OWNER_NONSPINNABLE; + rcu_read_unlock(); - if (need_resched() || !owner_on_cpu(owner)) { + if (!same_owner) { + state = rwsem_owner_state(new, new_flags, nonspinnable); + break; + } + + if (state == OWNER_NONSPINNABLE) + break; + + if (need_resched()) { state = OWNER_NONSPINNABLE; break; } cpu_relax(); } - rcu_read_unlock(); return state; } -- 2.28.0