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Sun, 1 Mar 2026 12:59:53 +0000 (GMT) From: Nilay Shroff To: linux-block@vger.kernel.org Cc: axboe@kernel.dk, hch@lst.de, ming.lei@redhat.com, yi.zhang@redhat.com, yukuai@fnnas.com, shinichiro.kawasaki@wdc.com, gjoyce@ibm.com Subject: [PATCHv2] block: break pcpu_alloc_mutex dependency on freeze_lock Date: Sun, 1 Mar 2026 18:29:43 +0530 Message-ID: <20260301125951.582690-1-nilay@linux.ibm.com> X-Mailer: git-send-email 2.53.0 Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit X-TM-AS-GCONF: 00 X-Proofpoint-ORIG-GUID: fa7qgYpAwYlmC3yBTTex3DG_0o4FZdb0 X-Authority-Analysis: v=2.4 cv=b66/I9Gx c=1 sm=1 tr=0 ts=69a4384e cx=c_pps a=5BHTudwdYE3Te8bg5FgnPg==:117 a=5BHTudwdYE3Te8bg5FgnPg==:17 a=Yq5XynenixoA:10 a=VkNPw1HP01LnGYTKEx00:22 a=RnoormkPH1_aCDwRdu11:22 a=V8glGbnc2Ofi9Qvn3v5h:22 a=VwQbUJbxAAAA:8 a=pGLkceISAAAA:8 a=20KFwNOVAAAA:8 a=VnNF1IyMAAAA:8 a=0ddmZlBLnuk0VgvrfPYA:9 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMzAxMDExNCBTYWx0ZWRfX69Qf6A7vLB5s 3L67gRTEMqOw/Hmm/xVLp3s4tOCu719ARjXeMjXgAbZpBIO283wkf7IIlxXhnBU4l/sMTU/VaUi QQUaKS6MBddXQmerTC+mumxfvPC4mqmEdoMtVykUnliYPwxLKaX3lG2HKG72Ec7t2IypxcT8QF1 6UfT+LqnIjdhQhAuByBBtRbLEwv0PNSTPAK7Zm553RHBWwN5oRuYQJf/9gk3BbdUgrNszfGpDp0 /mmTvBaiTYxktbsw3M2z160h3Gr+0e1TC8R8NjHa9PlGkVlh2oN/MKyYNuWWr62yxVSRJYMud7S 5ag7w4WfzpvE8XTTCvVdtNkkMgMCFEy5VxTj81CVcZgpsCKMU4TWN1gL50jd5LL4sBgDWs0lu6g K43lTvF7RKm6YlqTyjlhQ6C1HIbYkk1XLHUAOjip6q7bGa7ndabQGZMslci8xbAH4daziBlT3Kn 6hvd1MV/pnomhkNFJXw== X-Proofpoint-GUID: fa7qgYpAwYlmC3yBTTex3DG_0o4FZdb0 X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1121,Hydra:6.1.51,FMLib:17.12.100.49 definitions=2026-03-01_01,2026-02-27_03,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 suspectscore=0 lowpriorityscore=0 phishscore=0 clxscore=1015 adultscore=0 bulkscore=0 impostorscore=0 malwarescore=0 spamscore=0 priorityscore=1501 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2602130000 definitions=main-2603010114 While nr_hw_update allocates tagset tags it acquires ->pcpu_alloc_mutex after ->freeze_lock is acquired or queue is frozen. This potentially creates a circular dependency involving ->fs_reclaim if reclaim is triggered simultaneously in a code path which first acquires ->pcpu_ alloc_mutex. As the queue is already frozen while nr_hw_queue update allocates tagsets, the reclaim can't forward progress and thus it could cause a potential deadlock as reported in lockdep splat[1]. Fix this by pre-allocating tagset tags before we freeze queue during nr_hw_queue update. Later the allocated tagset tags could be safely installed and used after queue is frozen. Reported-by: Yi Zhang Closes: https://lore.kernel.org/all/CAHj4cs8F=OV9s3La2kEQ34YndgfZP-B5PHS4Z8_b9euKG6J4mw@mail.gmail.com/ [1] Signed-off-by: Nilay Shroff --- Changes from v1: - Make blk_mq_prealloc_tag_set_tags() return the tags pointer directly, encoding error conditions using ERR_PTR(). Update caller to use IS_ERR() to distinguish between success and failure. (Yu Kuai) --- block/blk-mq.c | 45 ++++++++++++++++++++++++++++++--------------- 1 file changed, 30 insertions(+), 15 deletions(-) diff --git a/block/blk-mq.c b/block/blk-mq.c index 9af8c3dec3f6..a27bade45de3 100644 --- a/block/blk-mq.c +++ b/block/blk-mq.c @@ -4793,38 +4793,45 @@ static void blk_mq_update_queue_map(struct blk_mq_tag_set *set) } } -static int blk_mq_realloc_tag_set_tags(struct blk_mq_tag_set *set, - int new_nr_hw_queues) +static struct blk_mq_tags **blk_mq_prealloc_tag_set_tags( + struct blk_mq_tag_set *set, + int new_nr_hw_queues) { struct blk_mq_tags **new_tags; int i; if (set->nr_hw_queues >= new_nr_hw_queues) - goto done; + return NULL; new_tags = kcalloc_node(new_nr_hw_queues, sizeof(struct blk_mq_tags *), GFP_KERNEL, set->numa_node); if (!new_tags) - return -ENOMEM; + return ERR_PTR(-ENOMEM); if (set->tags) memcpy(new_tags, set->tags, set->nr_hw_queues * sizeof(*set->tags)); - kfree(set->tags); - set->tags = new_tags; for (i = set->nr_hw_queues; i < new_nr_hw_queues; i++) { - if (!__blk_mq_alloc_map_and_rqs(set, i)) { - while (--i >= set->nr_hw_queues) - __blk_mq_free_map_and_rqs(set, i); - return -ENOMEM; + if (blk_mq_is_shared_tags(set->flags)) + new_tags[i] = set->shared_tags; + else { + new_tags[i] = blk_mq_alloc_map_and_rqs(set, i, + set->queue_depth); + if (!new_tags[i]) + goto out_unwind; } cond_resched(); } -done: - set->nr_hw_queues = new_nr_hw_queues; - return 0; + return new_tags; +out_unwind: + while (--i >= set->nr_hw_queues) { + if (!blk_mq_is_shared_tags(set->flags)) + blk_mq_free_map_and_rqs(set, new_tags[i], i); + } + kfree(new_tags); + return ERR_PTR(-ENOMEM); } /* @@ -5113,6 +5120,7 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, unsigned int memflags; int i; struct xarray elv_tbl; + struct blk_mq_tags **new_tags; bool queues_frozen = false; lockdep_assert_held(&set->tag_list_lock); @@ -5147,11 +5155,18 @@ static void __blk_mq_update_nr_hw_queues(struct blk_mq_tag_set *set, if (blk_mq_elv_switch_none(q, &elv_tbl)) goto switch_back; + new_tags = blk_mq_prealloc_tag_set_tags(set, nr_hw_queues); + if (IS_ERR(new_tags)) + goto switch_back; + list_for_each_entry(q, &set->tag_list, tag_set_list) blk_mq_freeze_queue_nomemsave(q); queues_frozen = true; - if (blk_mq_realloc_tag_set_tags(set, nr_hw_queues) < 0) - goto switch_back; + if (new_tags) { + kfree(set->tags); + set->tags = new_tags; + } + set->nr_hw_queues = nr_hw_queues; fallback: blk_mq_update_queue_map(set); -- 2.53.0