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McKenney" To: linux-kernel@vger.kernel.org Cc: mingo@kernel.org, jiangshanlai@gmail.com, dipankar@in.ibm.com, akpm@linux-foundation.org, mathieu.desnoyers@efficios.com, josh@joshtriplett.org, tglx@linutronix.de, peterz@infradead.org, rostedt@goodmis.org, dhowells@redhat.com, edumazet@google.com, fweisbec@gmail.com, oleg@redhat.com, joel@joelfernandes.org, "Paul E . McKenney" Subject: [PATCH tip/core/rcu 16/27] rcu: Add comment documenting how rcu_seq_snap works Date: Mon, 25 Jun 2018 17:35:02 -0700 X-Mailer: git-send-email 2.17.1 In-Reply-To: <20180626003448.GA26209@linux.vnet.ibm.com> References: <20180626003448.GA26209@linux.vnet.ibm.com> X-TM-AS-GCONF: 00 x-cbid: 18062600-0052-0000-0000-000003040982 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00009255; HX=3.00000241; KW=3.00000007; PH=3.00000004; SC=3.00000266; SDB=6.01052336; UDB=6.00539469; IPR=6.00830262; MB=3.00021856; MTD=3.00000008; XFM=3.00000015; UTC=2018-06-26 00:33:16 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18062600-0053-0000-0000-00005D23DD94 Message-Id: <20180626003513.27812-16-paulmck@linux.vnet.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-06-25_11:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=1 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1806210000 definitions=main-1806260005 Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk 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) Signed-off-by: Paul E. McKenney --- 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 aa215d6355f8..60f089d08c47 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; -- 2.17.1