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=-2.4 required=3.0 tests=DKIM_SIGNED,DKIM_VALID, DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_PASS, URIBL_BLOCKED,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 34BE2C43144 for ; Thu, 28 Jun 2018 05:10:35 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D143626C7B for ; Thu, 28 Jun 2018 05:10:34 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (1024-bit key) header.d=joelfernandes.org header.i=@joelfernandes.org header.b="nt6Mo8IQ" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D143626C7B Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=joelfernandes.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 S933065AbeF1FKb (ORCPT ); Thu, 28 Jun 2018 01:10:31 -0400 Received: from mail-pl0-f67.google.com ([209.85.160.67]:44598 "EHLO mail-pl0-f67.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932329AbeF1FKa (ORCPT ); Thu, 28 Jun 2018 01:10:30 -0400 Received: by mail-pl0-f67.google.com with SMTP id m16-v6so2161921pls.11 for ; Wed, 27 Jun 2018 22:10:30 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=joelfernandes.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=ijMpWL981q+N7asYRk6SO6v6ov8dZ8gi1QPwnOjSjYg=; b=nt6Mo8IQ+I0WzCvuOKozunwzfy62JZ12WnvaNJ2H2TSJOgylNCgEvlIIBPVOvxxK/4 p2/fRrwHSAQteKQ3z/r77MThKcUu9c+Tq0UmOeCbZ6mAGYAUY3Ah4tEzmyzcP8YuDsPh TaOw4j1TQmCw9Xorhr5aJbitOI/vIDPl2CjM0= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=ijMpWL981q+N7asYRk6SO6v6ov8dZ8gi1QPwnOjSjYg=; b=KWDirURVC4OWd9FkZqnW7g4HjNY/RFx7NSnrRfkXHa8atcbFgXQYs00GCw/7Elynps o0zbyQh6zT7lx3pq2TVv9STHhUmhon38NCAPBjnS17OR/ZD8D8OjYcTfFNBTXdAkFsyi ArzYCjsK9iKl3I1/gD0emDbvopGcu0PEPTKCQWrfwy4okvJGNecs3220Q2nWI1wg4JDA XiXhbOlSJubpd6HB0hB1EZQ1IU4MhewvBx/7s69Dg4CIDl1eDVsJ17CaaEojck2y3fPB v0eEskcgl8h9WOtceVw4YkumPJieQAZQBX8Su8BlbdqkbyBNskW9+/JKeSAk2N5r/tIe GLLg== X-Gm-Message-State: APt69E1Yy1DAGvpbN4niyI4ItGhdXTAGzGd89m2UzzFEc4yyxQSZOdCI LNUF5JtfMUl5mdFCTyUNfQ67Cg== X-Google-Smtp-Source: ADUXVKLtPjYnClJduwr9DU0k254UQ5dHU436q6vKnn/FniskCLtDfwhQvHVd6SWzNtOTN5hwi/+bqg== X-Received: by 2002:a17:902:728a:: with SMTP id d10-v6mr8943171pll.192.1530162629728; Wed, 27 Jun 2018 22:10:29 -0700 (PDT) Received: from localhost ([2620:0:1000:1600:3122:ea9c:d178:eb]) by smtp.gmail.com with ESMTPSA id g10-v6sm8212512pfi.148.2018.06.27.22.10.28 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 27 Jun 2018 22:10:28 -0700 (PDT) Date: Wed, 27 Jun 2018 22:10:28 -0700 From: Joel Fernandes To: "Paul E. McKenney" Cc: Peter Zijlstra , 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, kernel-team@android.com Subject: Re: [PATCH tip/core/rcu 16/27] rcu: Add comment documenting how rcu_seq_snap works Message-ID: <20180628051028.GB79165@joelaf.mtv.corp.google.com> References: <20180626003448.GA26209@linux.vnet.ibm.com> <20180626003513.27812-16-paulmck@linux.vnet.ibm.com> <20180626173055.GJ2494@hirez.programming.kicks-ass.net> <20180627043913.GA177710@joelaf.mtv.corp.google.com> <20180627175436.GC3593@linux.vnet.ibm.com> <20180627182726.GA79165@joelaf.mtv.corp.google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20180627182726.GA79165@joelaf.mtv.corp.google.com> User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Wed, Jun 27, 2018 at 11:27:26AM -0700, Joel Fernandes wrote: [..] > > > > s = __ALIGN_MASK(s, RCU_SEQ_STATE_MASK); > > > > > > > > > > I agree with Peter's suggestions for both the verbiage reduction in the > > > comments in the header, as the new code he is proposing is more > > > self-documenting. I believe I proposed a big comment just because the code > > > wasn't self-documenting or obvious previously so needed an explanation. > > > > > > How would you like to proceed? Let me know what you guys decide, I am really > > > Ok with anything. If you guys agree, should I write a follow-up patch with > > > Peter's suggestion that applies on top of this one? Or do we want to drop > > > this one in favor of Peter's suggestion? > > > > Shortening the comment would be good, so please do that. Paul, Do you want to fold the below patch into the original one? Or do you prefer I resent the original patch fixed up? Following is the patch ontop of current 'dev' branch in your tree, with the excessive comments removed. Thanks to Peter for suggesting! ---8<----------------------- From: "Joel Fernandes (Google)" Subject: [PATCH] rcu: Remove excessive commentary on rcu_seq_snap There isn't a strong need to explain in excessive detail about rcu_seq_snap with an example. Remove unnecessary and redundant comments. Suggested-by: Peter Zijlstra Fixes: 9701945dd79e ("rcu: Add comment documenting how rcu_seq_snap works") Signed-off-by: Joel Fernandes (Google) --- kernel/rcu/rcu.h | 22 ---------------------- 1 file changed, 22 deletions(-) diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index 0af6ce6d8b66..4d04683c31b2 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -101,28 +101,6 @@ static inline void rcu_seq_end(unsigned long *sp) * 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. - * - * 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) */ static inline unsigned long rcu_seq_snap(unsigned long *sp) { -- 2.18.0.rc2.346.g013aa6912e-goog