From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753398AbeENRgx (ORCPT ); Mon, 14 May 2018 13:36:53 -0400 Received: from mx0a-001b2d01.pphosted.com ([148.163.156.1]:35446 "EHLO mx0a-001b2d01.pphosted.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752474AbeENRgv (ORCPT ); Mon, 14 May 2018 13:36:51 -0400 Date: Mon, 14 May 2018 10:38:16 -0700 From: "Paul E. McKenney" To: "Joel Fernandes (Google)" Cc: linux-kernel@vger.kernel.org, Josh Triplett , Steven Rostedt , Mathieu Desnoyers , Lai Jiangshan , byungchul.park@lge.com, kernel-team@android.com Subject: Re: [PATCH RFC 1/8] rcu: Add comment documenting how rcu_seq_snap works Reply-To: paulmck@linux.vnet.ibm.com References: <20180514031541.67247-1-joel@joelfernandes.org> <20180514031541.67247-2-joel@joelfernandes.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180514031541.67247-2-joel@joelfernandes.org> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18051417-0056-0000-0000-0000044E24CA X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009025; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000260; SDB=6.01032264; UDB=6.00527713; IPR=6.00811396; MB=3.00021110; MTD=3.00000008; XFM=3.00000015; UTC=2018-05-14 17:36:48 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18051417-0057-0000-0000-000008923850 Message-Id: <20180514173816.GA26088@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-05-14_04:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 impostorscore=0 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1709140000 definitions=main-1805140177 Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Sun, May 13, 2018 at 08:15:34PM -0700, 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. How about something like this? This function returns the earliest value of the grace-period sequence number that will indicate that a full grace period has elapsed since the current time. Once the grace-period sequence number has reached this value, it will be safe to invoke all callbacks that have been registered prior to the current time. This value is the current grace-period number plus two to the power of the number of low-order bits reserved for state, then rounded up to the next value in which the state bits are all zero. > + * > + * We do this with a single addition and masking. Please either fold this sentence into rest of the paragraph or add a blank line after it. > + * 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. 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. > + */ Having the explicit numbers is good, but please use RCU_SEQ_STATE_MASK=3, since that is the current value. One alternative (or perhaps addition) is to have a short table of numbers showing the mapping from *sp to the return value. (I started from such a table when writing this function, for whatever that is worth.) Thanx, Paul > static inline unsigned long rcu_seq_snap(unsigned long *sp) > { > unsigned long s; > -- > 2.17.0.441.gb46fe60e1d-goog >