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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 22/27] rcu: Make rcu_seq_diff() more exact Date: Mon, 25 Jun 2018 17:35:08 -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: 18062600-0060-0000-0000-000002827C1B 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=3.00021855; 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: 18062600-0061-0000-0000-00004593574B Message-Id: <20180626003513.27812-22-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 The current implementatation of rcu_seq_diff() follows tradition in providing a rough-and-ready approximation of the number of elapsed grace periods between the two rcu_seq values. However, this difference is used to flag RCU-failure "near misses", which can be a valuable debugging aid, so more exactitude would be an improvement. This commit therefore improves the accuracy of rcu_seq_diff(). Signed-off-by: Paul E. McKenney --- kernel/rcu/rcu.h | 15 ++++++++++++++- 1 file changed, 14 insertions(+), 1 deletion(-) diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h index 60f089d08c47..46f12d0fe319 100644 --- a/kernel/rcu/rcu.h +++ b/kernel/rcu/rcu.h @@ -180,7 +180,20 @@ static inline bool rcu_seq_new_gp(unsigned long old, unsigned long new) */ static inline unsigned long rcu_seq_diff(unsigned long new, unsigned long old) { - return (new - old) >> RCU_SEQ_CTR_SHIFT; + unsigned long rnd_diff; + + if (old == new) + return 0; + /* + * Compute the number of grace periods (still shifted up), plus + * one if either of new and old is not an exact grace period. + */ + rnd_diff = (new & ~RCU_SEQ_STATE_MASK) - + ((old + RCU_SEQ_STATE_MASK) & ~RCU_SEQ_STATE_MASK) + + ((new & RCU_SEQ_STATE_MASK) || (old & RCU_SEQ_STATE_MASK)); + if (ULONG_CMP_GE(RCU_SEQ_STATE_MASK, rnd_diff)) + return 1; /* Definitely no grace period has elapsed. */ + return ((rnd_diff - RCU_SEQ_STATE_MASK - 1) >> RCU_SEQ_CTR_SHIFT) + 2; } /* -- 2.17.1