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Tue, 9 Sep 2025 09:36:46 +0000 (GMT) Message-ID: <4a25f585-85be-4b58-99d3-9833bbeb4b3b@linux.ibm.com> Date: Tue, 9 Sep 2025 15:06:44 +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 08/10] blk-mq: fix potential deadlock while nr_requests grown 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> <20250908061533.3062917-9-yukuai1@huaweicloud.com> <484c552e-462f-488f-90bf-18fe5042f1bb@linux.ibm.com> <628d9525-dcef-2aaa-3434-64c7cdc0fb23@huaweicloud.com> Content-Language: en-US From: Nilay Shroff In-Reply-To: <628d9525-dcef-2aaa-3434-64c7cdc0fb23@huaweicloud.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjUwOTA2MDE5NSBTYWx0ZWRfX+MmiihXJ6Pii OuI/Ug5hSYIiNR3V/yPPL3Jga4On2kDqgCJ2NNu4cT3fxbdJOglZg/Uz2EwJqeHWp3fzT94uiJ5 zeTcujvDX3XKxyooAazZXbDsCvkkj8tEUxOC9/ido2GN3sFKc57iZTYbxF7/g68rFL444j1yZRw Ar/h1jo7GtiAiUWTTp3oSvCLhy4JH6lUmMF6hmxbbV4LtvrPObXmydbOx3Y99hfGRvsNV9Rs/nz JjlEpcDXpwbbYjVrtMQuAa3fwL750rH6gAfmZf9h5iU/xmSmwvGM1heXw1rfOLLlThzpfiD21vM 3+Nt3znSxNxBcnn+TZnGiycK3fJ5kee+BfGNMQHJBxXuRXiOh5IUqVRfE2gQaV0fJMoiREp7bCP sGSfSAT+ X-Proofpoint-ORIG-GUID: ds7QzfiKxqKIxHU7mBFHI0TEFpRAtZQO X-Proofpoint-GUID: ds7QzfiKxqKIxHU7mBFHI0TEFpRAtZQO X-Authority-Analysis: v=2.4 cv=StCQ6OO0 c=1 sm=1 tr=0 ts=68bff533 cx=c_pps a=GFwsV6G8L6GxiO2Y/PsHdQ==:117 a=GFwsV6G8L6GxiO2Y/PsHdQ==:17 a=IkcTkHD0fZMA:10 a=yJojWOMRYYMA:10 a=VwQbUJbxAAAA:8 a=VnNF1IyMAAAA:8 a=i0EeH86SAAAA:8 a=CvrVc3EHEBBFqsY-YQIA:9 a=3ZKOabzyN94A:10 a=QEXdDO2ut3YA:10 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 spamscore=0 malwarescore=0 bulkscore=0 clxscore=1015 adultscore=0 suspectscore=0 priorityscore=1501 impostorscore=0 phishscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2507300000 definitions=main-2509060195 On 9/9/25 1:07 PM, Yu Kuai wrote: > Hi, > > 在 2025/09/09 14:39, Nilay Shroff 写道: >> >> >> On 9/8/25 11:45 AM, Yu Kuai wrote: >>> From: Yu Kuai >>> >>> Allocate and free sched_tags while queue is freezed can deadlock[1], >>> this is a long term problem, hence allocate memory before freezing >>> queue and free memory after queue is unfreezed. >>> >>> [1] https://lore.kernel.org/all/0659ea8d-a463-47c8-9180-43c719e106eb@linux.ibm.com/ >>> Fixes: e3a2b3f931f5 ("blk-mq: allow changing of queue depth through sysfs") >>> >>> Signed-off-by: Yu Kuai >> [...] >> [...] >> >>> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c >>> index 7ea15bf68b4b..a0a7ebad378f 100644 >>> --- a/block/blk-sysfs.c >>> +++ b/block/blk-sysfs.c >>> @@ -64,11 +64,12 @@ static ssize_t queue_requests_show(struct gendisk *disk, char *page) >>>   static ssize_t >>>   queue_requests_store(struct gendisk *disk, const char *page, size_t count) >>>   { >>> -    unsigned long nr; >>> -    int ret, err; >>> -    unsigned int memflags; >>>       struct request_queue *q = disk->queue; >>>       struct blk_mq_tag_set *set = q->tag_set; >>> +    struct elevator_tags *et = NULL; >>> +    unsigned int memflags; >>> +    unsigned long nr; >>> +    int ret; >>>         ret = queue_var_store(&nr, page, count); >>>       if (ret < 0) >>> @@ -90,16 +91,24 @@ queue_requests_store(struct gendisk *disk, const char *page, size_t count) >>>           goto unlock; >>>       } >>>   +    if (q->elevator && nr > q->elevator->et->nr_requests) { >>> +        /* allocate memory before freezing queue to prevent deadlock */ >>> +        et = blk_mq_alloc_sched_tags(set, q->nr_hw_queues, nr); >>> +        if (!et) { >>> +            ret = -ENOMEM; >>> +            goto unlock; >>> +        } >>> +    } >>> + >> I think we should add a comment above explaining why is it safe >> to access q->elevator without holding ->elevator_lock. >> > > I already access q->elevator to check input nr from patch 4, and that's > why I add comments to explain switching elevator is serialized, is this > enough? > yes in patch 04/10 you moved the ->elevator_lock after the usual sanity checks. However when we run those sanity checks or the code in this patch where we have to access q->elevator, it's good to add a comment here in the code (not in commit). For reference, you may check blk_mq_alloc_sched_tags_batch. I think similar comment may be added here as well. Thanks, --Nilay