From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1751290AbeELAdJ (ORCPT ); Fri, 11 May 2018 20:33:09 -0400 Received: from mail-pl0-f66.google.com ([209.85.160.66]:41777 "EHLO mail-pl0-f66.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1750776AbeELAdI (ORCPT ); Fri, 11 May 2018 20:33:08 -0400 X-Google-Smtp-Source: AB8JxZqDP7rLgnD2WpNoFq7OAXHfXKPfYnEOOvNYbzutZ87ps1/1bqRZke1uuwT0iC59GO7AonyWnw== Date: Fri, 11 May 2018 17:33:06 -0700 From: "Joel Fernandes (Google)" To: linux-kernel@vger.kernel.org, paulmck@linux.vnet.ibm.com Subject: [PATCH] rcu: Add comment documenting how rcu_seq_snap works Message-ID: <20180512003306.GA170941@joelaf.mtv.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org rcu_seq_snap may be tricky for someone looking at it for the first time. Lets document how it works with an example to make it easier. Signed-off-by: Joel Fernandes (Google) --- kernel/rcu/rcu.h | 23 ++++++++++++++++++++++- 1 file changed, 22 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index 003671825d62..004ace3d22c2 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -91,7 +91,28 @@ static inline void rcu_seq_end(unsigned long *sp) WRITE_ONCE(*sp, rcu_seq_endval(sp)); } -/* Take a snapshot of the update side's sequence number. */ +/* + * Take a snapshot of the update side's sequence number. + * + * This function predicts what the grace period number will be the next + * time an RCU callback will be executed, given the current grace period's + * number. This can be gp+1 if RCU is idle, or gp+2 if a grace period is + * already in progress. + * + * We do this with a single addition and masking. + * For example, if RCU_SEQ_STATE_MASK=1 and the least significant bit (LSB) of + * the seq is used to track if a GP is in progress or not, its sufficient if we + * add (2+1) and mask with ~1. Lets see why with an example: + * + * Say the current seq is 6 which is 0x110 (gp is 3 and state bit is 0). + * To get the next GP number, we have to atleast add 0x10 to this (0x1 << 1) to + * account for the state bit. However, if the current seq is 7 (GP num is 3 + * and state bit is 1), then it means the current grace period is already + * in progress so the next the callback will run is at gp+2. To account for + * the extra +1, we just overflow the LSB by adding another 0x1 and masking + * with ~0x1. Incase no GP was in progress (RCU is idle), then the adding + * by 0x1 and masking will have no effect. This is calculated as below. + */ static inline unsigned long rcu_seq_snap(unsigned long *sp) { unsigned long s; -- 2.17.0.441.gb46fe60e1d-goog