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 vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 1D2EAC433F5 for ; Tue, 19 Apr 2022 00:04:15 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S236195AbiDSAGx (ORCPT ); Mon, 18 Apr 2022 20:06:53 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:52778 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S236928AbiDSAGJ (ORCPT ); Mon, 18 Apr 2022 20:06:09 -0400 Received: from dfw.source.kernel.org (dfw.source.kernel.org [IPv6:2604:1380:4641:c500::1]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 3C220252B7; Mon, 18 Apr 2022 17:03:28 -0700 (PDT) Received: from smtp.kernel.org (relay.kernel.org [52.25.139.140]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by dfw.source.kernel.org (Postfix) with ESMTPS id 83B53612DB; Tue, 19 Apr 2022 00:03:26 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 1EA40C385BF; Tue, 19 Apr 2022 00:03:25 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1650326605; bh=dq5EhGRV2Ky6K2MIwwoj/ydAY0Nn8LeiyWw4AjAjknk=; h=From:To:Cc:Subject:Date:In-Reply-To:References:From; b=kdPxWn0X8WovhPqBfABGi2JA5idnO5voDpqxlPRx/LYFSXMhr8uJ0igVjL5Olfo3n 9YXvZyZp32vewI01Dyz53JtD3GqYT9kHWXSlxh25qmeWSrnPKO8ISqn9wPe7JkziSP RbwRRJGedYWe+k/FNluGQPItvRPOUXnrDFtYhh1K9Nj3rt02h2aSqvmqBlXZ78xsD5 CE0iQScP2CqwzR4p6o2wzCkWv/QRY8JZNbeJInDLzSaROdWzf0rgSrecuPGSHmehyE 27CtMiAPHe+IUYa7Ssp5mcfCbyaizditBRh/cXDAYoP+odjZgH6Q+pcceD5Tv7EjXm kB5yiNQGl1t+w== Received: by paulmck-ThinkPad-P17-Gen-1.home (Postfix, from userid 1000) id 67C505C2DEA; Mon, 18 Apr 2022 17:03:24 -0700 (PDT) From: "Paul E. McKenney" To: rcu@vger.kernel.org Cc: linux-kernel@vger.kernel.org, kernel-team@fb.com, rostedt@goodmis.org, "Paul E. McKenney" , Neeraj Upadhyay Subject: [PATCH rcu 15/21] srcu: Explain srcu_funnel_gp_start() call to list_add() is safe Date: Mon, 18 Apr 2022 17:03:16 -0700 Message-Id: <20220419000322.3948903-15-paulmck@kernel.org> X-Mailer: git-send-email 2.31.1.189.g2e36527f23 In-Reply-To: <20220419000315.GA3948789@paulmck-ThinkPad-P17-Gen-1> References: <20220419000315.GA3948789@paulmck-ThinkPad-P17-Gen-1> MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This commit adds a comment explaining why an unprotected call to list_add() from srcu_funnel_gp_start() can be safe. TL;DR: It is only called during very early boot when we don't have no steeking concurrency! Co-developed-by: Neeraj Upadhyay Signed-off-by: Neeraj Upadhyay Signed-off-by: Paul E. McKenney --- kernel/rcu/srcutree.c | 6 ++++++ 1 file changed, 6 insertions(+) diff --git a/kernel/rcu/srcutree.c b/kernel/rcu/srcutree.c index 7209fd95dde9..64993a172cff 100644 --- a/kernel/rcu/srcutree.c +++ b/kernel/rcu/srcutree.c @@ -776,6 +776,12 @@ static void srcu_funnel_gp_start(struct srcu_struct *ssp, struct srcu_data *sdp, rcu_seq_state(ssp->srcu_gp_seq) == SRCU_STATE_IDLE) { WARN_ON_ONCE(ULONG_CMP_GE(ssp->srcu_gp_seq, ssp->srcu_gp_seq_needed)); srcu_gp_start(ssp); + + // And how can that list_add() in the "else" clause + // possibly be safe for concurrent execution? Well, + // it isn't. And it does not have to be. After all, it + // can only be executed during early boot when there is only + // the one boot CPU running with interrupts still disabled. if (likely(srcu_init_done)) queue_delayed_work(rcu_gp_wq, &ssp->work, srcu_get_delay(ssp)); -- 2.31.1.189.g2e36527f23