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Tue, 9 Sep 2025 06:52:50 +0000 (GMT) Message-ID: Date: Tue, 9 Sep 2025 12:22:49 +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 for-6.18/block 04/10] blk-mq: convert to serialize updating nr_requests with update_nr_hwq_lock To: Yu Kuai , ming.lei@redhat.com, axboe@kernel.dk Cc: linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, yi.zhang@huawei.com, yangerkun@huawei.com, johnny.chenyi@huawei.com, "yukuai (C)" References: <20250908061533.3062917-1-yukuai1@huaweicloud.com> <20250908061533.3062917-5-yukuai1@huaweicloud.com> <9708abeb-7677-4c0e-931b-7ca5fe0a0242@linux.ibm.com> <329ca336-21f6-e686-0446-b3ae9a46f4c9@huaweicloud.com> Content-Language: en-US From: Nilay Shroff In-Reply-To: <329ca336-21f6-e686-0446-b3ae9a46f4c9@huaweicloud.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: k-rbBzYtrpefH5p53gWVV-A3w0uzCIuN X-Proofpoint-GUID: k-rbBzYtrpefH5p53gWVV-A3w0uzCIuN X-Authority-Analysis: v=2.4 cv=EYDIQOmC c=1 sm=1 tr=0 ts=68bfced8 cx=c_pps a=AfN7/Ok6k8XGzOShvHwTGQ==:117 a=AfN7/Ok6k8XGzOShvHwTGQ==:17 a=IkcTkHD0fZMA:10 a=yJojWOMRYYMA:10 a=i0EeH86SAAAA:8 a=fbS70rplHMDwEZbDWOYA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwOTA2MDAyMCBTYWx0ZWRfX0vs/TF6Vifqw YD8/3Lx3IqMrs2lH1Rwx/RSy49r2JrJ2rsE7fc9sWaGzHbsqdVQqcjRK+d/XMHDCRmUJiQfYIKf ym4vElS4Tl+2TUM3x849zzGD2dczvjuyxXPrT7hP2dwXd1aXHg31Ch7EQ9LzNjcbTEpfRV7TIcE s4XAms29cYVM2/8EEu1XrZqcDxdDjMt9QQMrJpAtfU2MbZ4IqKChGU5U5wFgQDkIsvnIrXNgcLP zm9NVmdqrvyWNviwYoWSzbR8/dcAMfiBz78LrWkYtYw9xjVUXtw6/+EY5TF5qamWcewRMcSywGN yoX8UKOlMMVraVUQXpN1ffBNHg+dOIf31r4k6q2/P4F+6VL6OBgpoNmdJZy7GJMXJfjQpcGMDJk Q5Rwcg0I X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1117,Hydra:6.1.9,FMLib:17.12.80.40 definitions=2025-09-08_06,2025-09-08_02,2025-03-28_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 malwarescore=0 adultscore=0 suspectscore=0 spamscore=0 impostorscore=0 priorityscore=1501 phishscore=0 clxscore=1015 bulkscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2507300000 definitions=main-2509060020 On 9/9/25 12:08 PM, Yu Kuai wrote: > Hi, > > 在 2025/09/09 14:29, Nilay Shroff 写道: >> >> >> On 9/8/25 11:45 AM, Yu Kuai wrote: >>> From: Yu Kuai >>> >>> request_queue->nr_requests can be changed by: >>> >>> a) switching elevator by update nr_hw_queues >>> b) switching elevator by elevator sysfs attribute >>> c) configue queue sysfs attribute nr_requests >>> >>> Current lock order is: >>> >>> 1) update_nr_hwq_lock, case a,b >>> 2) freeze_queue >>> 3) elevator_lock, cas a,b,c >>> >>> And update nr_requests is seriablized by elevator_lock() already, >>> however, in the case c), we'll have to allocate new sched_tags if >>> nr_requests grow, and do this with elevator_lock held and queue >>> freezed has the risk of deadlock. >>> >>> Hence use update_nr_hwq_lock instead, make it possible to allocate >>> memory if tags grow, meanwhile also prevent nr_requests to be changed >>> concurrently. >>> >>> Signed-off-by: Yu Kuai >>> --- >>>   block/blk-sysfs.c | 12 +++++++++--- >>>   1 file changed, 9 insertions(+), 3 deletions(-) >>> >>> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c >>> index f99519f7a820..7ea15bf68b4b 100644 >>> --- a/block/blk-sysfs.c >>> +++ b/block/blk-sysfs.c >>> @@ -68,13 +68,14 @@ queue_requests_store(struct gendisk *disk, const char *page, size_t count) >>>       int ret, err; >>>       unsigned int memflags; >>>       struct request_queue *q = disk->queue; >>> +    struct blk_mq_tag_set *set = q->tag_set; >>>         ret = queue_var_store(&nr, page, count); >>>       if (ret < 0) >>>           return ret; >>>   -    memflags = blk_mq_freeze_queue(q); >>> -    mutex_lock(&q->elevator_lock); >>> +    /* serialize updating nr_requests with switching elevator */ >>> +    down_write(&set->update_nr_hwq_lock); >>>   >> For serializing update of nr_requests with switching elevator, >> we should use disable_elv_switch(). So with this change we >> don't need to acquire ->update_nr_hwq_lock in writer context >> while running blk_mq_update_nr_requests but instead it can run >> acquiring ->update_nr_hwq_lock in the reader context. >> >> So the code flow should be, >> >> disable_elv_switch  => this would set QUEUE_FLAG_NO_ELV_SWITCH >> ... >> down_read ->update_nr_hwq_lock >> acquire ->freeze_lock >> acquire ->elevator_lock; >> ... >> ... >> release ->elevator_lock; >> release ->freeze_lock >> >> clear QUEUE_FLAG_NO_ELV_SWITCH >> up_read ->update_nr_hwq_lock >> > > Yes, this make sense, however, there is also an implied condition that > we should serialize queue_requests_store() with itself, what if a > concurrent caller succeed the disable_elv_switch() before the > down_read() in this way? > > t1: > disable_elv_switch >         t2: >         disable_elv_switch > > down_read    down_read > I believe this is already protected with the kernfs internal mutex locks. So you shouldn't be able to run two sysfs store operations concurrently on the same attribute file. Thanks, --Nilay