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=-2.1 required=3.0 tests=DKIM_SIGNED, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS,T_DKIM_INVALID, USER_AGENT_MUTT 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 14880C43142 for ; Tue, 26 Jun 2018 17:31:10 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id C52A126E8F for ; Tue, 26 Jun 2018 17:31:09 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=fail reason="signature verification failed" (2048-bit key) header.d=infradead.org header.i=@infradead.org header.b="hyg2SFCf" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org C52A126E8F Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=infradead.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S933723AbeFZRbI (ORCPT ); Tue, 26 Jun 2018 13:31:08 -0400 Received: from bombadil.infradead.org ([198.137.202.133]:53346 "EHLO bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751102AbeFZRbH (ORCPT ); Tue, 26 Jun 2018 13:31:07 -0400 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=bombadil.20170209; h=In-Reply-To:Content-Type:MIME-Version :References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Id: List-Help:List-Unsubscribe:List-Subscribe:List-Post:List-Owner:List-Archive; bh=2QPWdQq/AHbNzeE6fkifLk607RtW337QFXuOQNKLDv0=; b=hyg2SFCfFF0P0AdZTPtww4QZg ZBWBPnBL9RRuhtl12z5yZ9bJfaC9T964VnB7YLjcCOEc7TTYJOuB4qLI2J4l9UJ+QbFlx2RfXfueZ qefLNHY1LpNymibwRCxhmD+BmKNw/Vi5UDbiQiwdgl51y08SggmVp2cKZW2313COft7O2nnzkB2O4 +228MmPpLKYu66FaKuIKDKrkutUZe71CqN/LY0LjOnrJ4aDdzErYH0fNxJ7dWkKgHQYwOm5oMYkPs 3iv6XbTOlL/MbusLJ/AGSQmH3szOS5Po2cg/Acf+UF3CGogra4hOiFY2/1jP3NzriVhjHzMa8W2PK eVznBZFNw==; Received: from j217100.upc-j.chello.nl ([24.132.217.100] helo=hirez.programming.kicks-ass.net) by bombadil.infradead.org with esmtpsa (Exim 4.90_1 #2 (Red Hat Linux)) id 1fXrnt-0000eI-V1; Tue, 26 Jun 2018 17:30:58 +0000 Received: by hirez.programming.kicks-ass.net (Postfix, from userid 1000) id BEDD62029FA0A; Tue, 26 Jun 2018 19:30:55 +0200 (CEST) Date: Tue, 26 Jun 2018 19:30:55 +0200 From: Peter Zijlstra To: "Paul E. McKenney" Cc: linux-kernel@vger.kernel.org, mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org Subject: Re: [PATCH tip/core/rcu 16/27] rcu: Add comment documenting how rcu_seq_snap works Message-ID: <20180626173055.GJ2494@hirez.programming.kicks-ass.net> References: <20180626003448.GA26209@linux.vnet.ibm.com> <20180626003513.27812-16-paulmck@linux.vnet.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180626003513.27812-16-paulmck@linux.vnet.ibm.com> User-Agent: Mutt/1.10.0 (2018-05-17) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Mon, Jun 25, 2018 at 05:35:02PM -0700, Paul E. McKenney wrote: > +/* > + * rcu_seq_snap - Take a snapshot of the update side's sequence number. > + * > + * 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. If you complete that by saying _why_ you need to round up there, then the below verbiage is completely redundant. > + * In the current design, RCU_SEQ_STATE_MASK=3 and the least significant bit of > + * the seq is used to track if a GP is in progress or not. Given this, it is > + * sufficient if we add (6+1) and mask with ~3 to get the next GP. Let's see > + * why with an example: > + * > + * Say the current seq is 12 which is 0b1100 (GP is 3 and state bits are 0b00). > + * To get to the next GP number of 4, we have to add 0b100 to this (0x1 << 2) > + * to account for the shift due to 2 state bits. Now, if the current seq is > + * 13 (GP is 3 and state bits are 0b01), then it means the current grace period > + * is already in progress so the next GP that a future call back will be queued > + * to run at is GP+2 = 5, not 4. To account for the extra +1, we just overflow > + * the 2 lower bits by adding 0b11. In case the lower bit was set, the overflow > + * will cause the extra +1 to the GP, along with the usual +1 explained before. > + * This gives us GP+2. Finally we mask the lower to bits by ~0x3 in case the > + * overflow didn't occur. This masking is needed because in case RCU was idle > + * (no GP in progress so lower 2 bits are 0b00), then the overflow of the lower > + * 2 state bits wouldn't occur, so we mask to zero out those lower 2 bits. > + * > + * In other words, the next seq can be obtained by (0b11 + 0b100) & (~0b11) > + * which can be generalized to: > + * seq + (RCU_SEQ_STATE_MASK + (RCU_SEQ_STATE_MASK + 1)) & (~RCU_SEQ_STATE_MASK) > + */ Is the below not much simpler: > static inline unsigned long rcu_seq_snap(unsigned long *sp) > { > unsigned long s; s = smp_load_aquire(sp); /* Add one GP */ s += 1 << RCU_SEQ_CTR_SHIFT; /* Complete any pending state by rounding up */ s = __ALIGN_MASK(s, RCU_SEQ_STATE_MASK); return s; }