From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754176AbeEWGjR (ORCPT ); Wed, 23 May 2018 02:39:17 -0400 Received: from mail-pl0-f68.google.com ([209.85.160.68]:40206 "EHLO mail-pl0-f68.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753902AbeEWGif (ORCPT ); Wed, 23 May 2018 02:38:35 -0400 X-Google-Smtp-Source: AB8JxZoHRx/3mE2XIsSXX/H0+uuUl63jD14yLJIWDRjhA9PDAK6gqFspD6Y4wWBk0FSyelbkWC5xnA== From: Joel Fernandes X-Google-Original-From: Joel Fernandes To: linux-kernel@vger.kernel.org Cc: "Joel Fernandes (Google)" , Boqun Feng , byungchul.park@lge.com, Ingo Molnar , Josh Triplett , kernel-team@android.com, Lai Jiangshan , Mathieu Desnoyers , Paul McKenney , Peter Zilstra , Steven Rostedt Subject: [PATCH 2/4] rcu: Add comment documenting how rcu_seq_snap works Date: Tue, 22 May 2018 23:38:13 -0700 Message-Id: <20180523063815.198302-3-joel@joelfernandes.org> X-Mailer: git-send-email 2.17.0.441.gb46fe60e1d-goog In-Reply-To: <20180523063815.198302-1-joel@joelfernandes.org> References: <20180523063815.198302-1-joel@joelfernandes.org> Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: "Joel Fernandes (Google)" rcu_seq_snap may be tricky to decipher. Lets document how it works with an example to make it easier. Signed-off-by: Joel Fernandes (Google) --- kernel/rcu/rcu.h | 34 +++++++++++++++++++++++++++++++++- 1 file changed, 33 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index 0453a7d12b3f..5819a37f19fb 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -91,7 +91,39 @@ 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. */ +/* + * 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. + * + * 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) { unsigned long s; -- 2.17.0.441.gb46fe60e1d-goog