From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752129AbeENDrk (ORCPT ); Sun, 13 May 2018 23:47:40 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:44902 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751847AbeENDrj (ORCPT ); Sun, 13 May 2018 23:47:39 -0400 Subject: Re: [PATCH RFC 1/8] rcu: Add comment documenting how rcu_seq_snap works To: "Joel Fernandes (Google)" , linux-kernel@vger.kernel.org Cc: "Paul E. McKenney" , Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , byungchul.park@lge.com, kernel-team@android.com References: <20180514031541.67247-1-joel@joelfernandes.org> <20180514031541.67247-2-joel@joelfernandes.org> From: Randy Dunlap Message-ID: Date: Sun, 13 May 2018 20:47:24 -0700 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:52.0) Gecko/20100101 Thunderbird/52.7.0 MIME-Version: 1.0 In-Reply-To: <20180514031541.67247-2-joel@joelfernandes.org> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 05/13/2018 08:15 PM, Joel Fernandes (Google) wrote: > 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 | 24 +++++++++++++++++++++++- > 1 file changed, 23 insertions(+), 1 deletion(-) > > diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h > index 003671825d62..fc3170914ac7 100644 > --- a/kernel/rcu/rcu.h > +++ b/kernel/rcu/rcu.h > @@ -91,7 +91,29 @@ 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 it's > + * add (2+1) and mask with ~1. Let's see why with an example: > + * > + * Say the current seq is 6 which is 0b110 (gp is 3 and state bit is 0). > + * To get the next GP number, we have to at least add 0b10 to this (0x1 << 1) > + * to account for the state bit. However, if the current seq is 7 (gp is 3 and > + * state bit is 1), then it means the current grace period is already in > + * progress so the next time the callback will run is at the end of grace > + * period number gp+2. To account for the extra +1, we just overflow the LSB by > + * adding another 0x1 and masking with ~0x1. In case no GP was in progress (RCU > + * is idle), then the addition of the extra 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; > -- ~Randy