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Win64; x64; rv:91.0) Gecko/20100101 Thunderbird/91.13.1 Subject: Re: [PATCH v3 4/5] locking/rwsem: Enable direct rwsem lock handoff Content-Language: en-US To: Hillf Danton , Waiman Long CC: Peter Zijlstra , Ingo Molnar , Will Deacon , Boqun Feng , , , =?UTF-8?B?VGluZzExIFdhbmcg546L5am3?= References: <20221017211356.333862-1-longman@redhat.com> <20221018111424.1007-1-hdanton@sina.com> From: Mukesh Ojha In-Reply-To: <20221018111424.1007-1-hdanton@sina.com> Content-Type: text/plain; charset="UTF-8"; format=flowed Content-Transfer-Encoding: 7bit X-Originating-IP: [10.80.80.8] X-ClientProxiedBy: nasanex01b.na.qualcomm.com (10.46.141.250) To nasanex01c.na.qualcomm.com (10.45.79.139) X-QCInternal: smtphost X-QCInternal: smtphost X-Proofpoint-Virus-Version: vendor=nai engine=6200 definitions=5800 signatures=585085 X-Proofpoint-Virus-Version: vendor=nai engine=6200 definitions=5800 signatures=585085 X-Proofpoint-GUID: U1HOZlofg8wKd-xITAVd5j7J61gQ6LbD X-Proofpoint-ORIG-GUID: U1HOZlofg8wKd-xITAVd5j7J61gQ6LbD X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.205,Aquarius:18.0.895,Hydra:6.0.545,FMLib:17.11.122.1 definitions=2022-10-18_04,2022-10-18_01,2022-06-22_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 impostorscore=0 malwarescore=0 mlxscore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 mlxlogscore=721 priorityscore=1501 adultscore=0 lowpriorityscore=0 clxscore=1015 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.12.0-2209130000 definitions=main-2210180080 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hi, On 10/18/2022 4:44 PM, Hillf Danton wrote: > On 17 Oct 2022 17:13:55 -0400 Waiman Long >> @@ -1067,13 +1119,33 @@ rwsem_down_read_slowpath(struct rw_semaphore *sem, long count, unsigned int stat >> return sem; >> } >> adjustment += RWSEM_FLAG_WAITERS; >> + } else if ((count & RWSEM_FLAG_HANDOFF) && >> + ((count & RWSEM_LOCK_MASK) == RWSEM_READER_BIAS)) { > > Could a couple of CPUs go read slow path in parallel? > >> + /* >> + * If the waiter to be handed off is a reader, this reader >> + * can piggyback on top of top of that. >> + */ >> + if (rwsem_first_waiter(sem)->type == RWSEM_WAITING_FOR_READ) >> + adjustment = 0; >> + rwsem_handoff(sem, adjustment, &wake_q); >> + >> + if (!adjustment) { >> + raw_spin_unlock_irq(&sem->wait_lock); >> + wake_up_q(&wake_q); >> + return sem; >> + } >> + adjustment = 0; >> } >> rwsem_add_waiter(sem, &waiter); > > Why can this acquirer pigyback without becoming a waiter? > >> >> - /* we're now waiting on the lock, but no longer actively locking */ >> - count = atomic_long_add_return(adjustment, &sem->count); >> - >> - rwsem_cond_wake_waiter(sem, count, &wake_q); >> + if (adjustment) { >> + /* >> + * We are now waiting on the lock with no handoff, but no >> + * longer actively locking. >> + */ >> + count = atomic_long_add_return(adjustment, &sem->count); >> + rwsem_cond_wake_waiter(sem, count, &wake_q); >> + } >> raw_spin_unlock_irq(&sem->wait_lock); >> >> if (!wake_q_empty(&wake_q)) >> @@ -1120,7 +1192,6 @@ static struct rw_semaphore __sched * >> rwsem_down_write_slowpath(struct rw_semaphore *sem, int state) >> { >> struct rwsem_waiter waiter; >> - int null_owner_retries; > > This reverts 2/5 and feel free to drop it directly. I think, he intents to tag the first two patches to go to stable branches. -Mukesh > > Hillf