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Sat, 19 Apr 2025 11:20:17 +0000 (GMT) Message-ID: <094c312a-38ec-4c5b-9db3-2269c37de36a@linux.ibm.com> Date: Sat, 19 Apr 2025 16:50:15 +0530 Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH V2 07/20] block: prevent adding/deleting disk during updating nr_hw_queues To: Ming Lei , Jens Axboe , linux-block@vger.kernel.org Cc: Shinichiro Kawasaki , =?UTF-8?Q?Thomas_Hellstr=C3=B6m?= , Christoph Hellwig References: <20250418163708.442085-1-ming.lei@redhat.com> <20250418163708.442085-8-ming.lei@redhat.com> Content-Language: en-US From: Nilay Shroff In-Reply-To: <20250418163708.442085-8-ming.lei@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: zdEXIlbwnBJIqAYYBWyx-64Y8JQHl7Ql X-Proofpoint-ORIG-GUID: zdEXIlbwnBJIqAYYBWyx-64Y8JQHl7Ql X-Authority-Analysis: v=2.4 cv=I8llRMgg c=1 sm=1 tr=0 ts=680386f5 cx=c_pps a=aDMHemPKRhS1OARIsFnwRA==:117 a=aDMHemPKRhS1OARIsFnwRA==:17 a=IkcTkHD0fZMA:10 a=XR8D0OoHHMoA:10 a=VwQbUJbxAAAA:8 a=VnNF1IyMAAAA:8 a=20KFwNOVAAAA:8 a=R2Tci1Go_5vh-wF9Tf0A:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1095,Hydra:6.0.680,FMLib:17.12.68.34 definitions=2025-04-19_04,2025-04-17_01,2024-11-22_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 adultscore=0 priorityscore=1501 phishscore=0 malwarescore=0 suspectscore=0 clxscore=1015 lowpriorityscore=0 spamscore=0 mlxlogscore=999 impostorscore=0 bulkscore=0 mlxscore=0 classifier=spam authscore=0 authtc=n/a authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2502280000 definitions=main-2504190090 On 4/18/25 10:06 PM, Ming Lei wrote: > Both adding/deleting disk code are reader of `nr_hw_queues`, so we can't > allow them in-progress when updating nr_hw_queues, kernel panic and > kasan has been reported in [1]. > > Prevent adding/deleting disk during updating nr_hw_queues by setting > set->updating_nr_hwq, and use SRCU to fail & retry to add/delete disk. > > This way avoids lot of trouble. > > Reported-by: Nilay Shroff > Closes: https://lore.kernel.org/linux-block/a5896cdb-a59a-4a37-9f99-20522f5d2987@linux.ibm.com/ > Signed-off-by: Ming Lei > --- [...] [...] > > +static int retry_on_updating_nr_hwq(struct gendisk_data *data, > + int (*cb)(struct gendisk_data *data)) > +{ > + struct gendisk *disk = data->disk; > + struct blk_mq_tag_set *set; > + > + if (!queue_is_mq(disk->queue)) > + return cb(data); > + > + set = disk->queue->tag_set; > + do { > + int idx, ret; > + > + idx = srcu_read_lock(&set->update_nr_hwq_srcu); > + if (set->updating_nr_hwq) { > + srcu_read_unlock(&set->update_nr_hwq_srcu, idx); > + goto wait; > + } > + ret = cb(data); > + srcu_read_unlock(&set->update_nr_hwq_srcu, idx); > + return ret; > + wait: > + wait_event_interruptible(set->update_nr_hwq_wq, > + !set->updating_nr_hwq); > + } while (true); > +} > + Yes using srcu with wait event would fix this. However after looking through changes it seems to me that the current changes are now modeled as below: - Writer(blk_mq_update_nr_hw_queues) must wait until all readers are done (or prior SRCU read-side critical-section complete) - Readers (add/del disk) must wait until a writer(blk_mq_update_nr_hw_queues) finishes And IMO, this behavior is best modeled with a reader-writer lock, rather than (S)RCU. In my view, RCU is optimized for read-heavy workloads with: - Non-blocking readers - Deferred updates by writers - Writers don’t block readers and readers don’t block writers. So I believe the simpler choice shall be using reader-writer lock instead of srcu. With reader-writer lock we should be also able to get away with implementing wait queue for add/del disk. Moreover, the reader-writer lock shall also work well with the another reader elv_iosched_store. Thanks, --Nilay