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Tue, 22 Apr 2025 06:15:00 +0000 (GMT) Message-ID: <286f9d0e-b782-4062-b0eb-cba6fa81e388@linux.ibm.com> Date: Tue, 22 Apr 2025 11:44:59 +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 20/20] block: move wbt_enable_default() out of queue freezing from sched ->exit() To: Ming Lei Cc: Jens Axboe , linux-block@vger.kernel.org, Shinichiro Kawasaki , =?UTF-8?Q?Thomas_Hellstr=C3=B6m?= , Christoph Hellwig References: <20250418163708.442085-1-ming.lei@redhat.com> <20250418163708.442085-21-ming.lei@redhat.com> <261d7b81-e611-47f4-ad55-6f7716c278c7@linux.ibm.com> Content-Language: en-US From: Nilay Shroff In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: -7PU0W34OrjwfKqQGHq_F5-L8oLsyIr9 X-Proofpoint-ORIG-GUID: -7PU0W34OrjwfKqQGHq_F5-L8oLsyIr9 X-Authority-Analysis: v=2.4 cv=CuO/cm4D c=1 sm=1 tr=0 ts=680733ea cx=c_pps a=bLidbwmWQ0KltjZqbj+ezA==:117 a=bLidbwmWQ0KltjZqbj+ezA==:17 a=IkcTkHD0fZMA:10 a=XR8D0OoHHMoA:10 a=20KFwNOVAAAA:8 a=wgIqA3w2QWIHfQYZNoAA:9 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=2025-04-22_03,2025-04-21_02,2024-11-22_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 clxscore=1015 spamscore=0 priorityscore=1501 lowpriorityscore=0 suspectscore=0 bulkscore=0 adultscore=0 impostorscore=0 malwarescore=0 phishscore=0 mlxlogscore=999 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-2504220045 On 4/21/25 12:57 PM, Ming Lei wrote: > On Sat, Apr 19, 2025 at 08:09:04PM +0530, Nilay Shroff wrote: >> >> >> On 4/18/25 10:07 PM, Ming Lei wrote: >>> scheduler's ->exit() is called with queue frozen and elevator lock is held, and >>> wbt_enable_default() can't be called with queue frozen, otherwise the >>> following lockdep warning is triggered: >>> >>> #6 (&q->rq_qos_mutex){+.+.}-{4:4}: >>> #5 (&eq->sysfs_lock){+.+.}-{4:4}: >>> #4 (&q->elevator_lock){+.+.}-{4:4}: >>> #3 (&q->q_usage_counter(io)#3){++++}-{0:0}: >>> #2 (fs_reclaim){+.+.}-{0:0}: >>> #1 (&sb->s_type->i_mutex_key#3){+.+.}-{4:4}: >>> #0 (&q->debugfs_mutex){+.+.}-{4:4}: >>> >>> Fix the issue by moving wbt_enable_default() out of bfq's exit(), and >>> call it from elevator_change_done(). >>> >>> Signed-off-by: Ming Lei >>> --- >>> block/bfq-iosched.c | 2 +- >>> block/elevator.c | 5 +++++ >>> block/elevator.h | 1 + >>> 3 files changed, 7 insertions(+), 1 deletion(-) >>> >>> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c >>> index 40e4106a71e7..310ce1d8c41e 100644 >>> --- a/block/bfq-iosched.c >>> +++ b/block/bfq-iosched.c >>> @@ -7211,7 +7211,7 @@ static void bfq_exit_queue(struct elevator_queue *e) >>> >>> blk_stat_disable_accounting(bfqd->queue); >>> blk_queue_flag_clear(QUEUE_FLAG_DISABLE_WBT, bfqd->queue); >>> - wbt_enable_default(bfqd->queue->disk); >>> + set_bit(ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT, &e->flags); >>> >>> kfree(bfqd); >>> } >>> diff --git a/block/elevator.c b/block/elevator.c >>> index 8652fe45a2db..378553fce5d8 100644 >>> --- a/block/elevator.c >>> +++ b/block/elevator.c >>> @@ -687,8 +687,13 @@ int elevator_change_done(struct request_queue *q, struct elv_change_ctx *ctx) >>> int ret = 0; >>> >>> if (ctx->old) { >>> + bool enable_wbt = test_bit(ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT, >>> + &ctx->old->flags); >>> + >>> elv_unregister_queue(q, ctx->old); >>> kobject_put(&ctx->old->kobj); >>> + if (enable_wbt) >>> + wbt_enable_default(q->disk); >>> } >>> if (ctx->new) { >>> ret = elv_register_queue(q, ctx->new, ctx->uevent); >>> diff --git a/block/elevator.h b/block/elevator.h >>> index 486be0690499..b14c611c74b6 100644 >>> --- a/block/elevator.h >>> +++ b/block/elevator.h >>> @@ -122,6 +122,7 @@ struct elevator_queue >>> >>> #define ELEVATOR_FLAG_REGISTERED 0 >>> #define ELEVATOR_FLAG_DYING 1 >>> +#define ELEVATOR_FLAG_ENABLE_WBT_ON_EXIT 2 >>> >>> /* Holding context data for changing elevator */ >>> struct elv_change_ctx { >> >> It seems invoking wbt_enable_default from elevator_change_done could probably >> still race with ioc_qos_write or queue_wb_lat_store. Both ioc_qos_write and >> queue_wb_lat_store run with ->freeze_lock and ->elevator_lock protection. > > Actually wbt_enable_default() and wbt_init() needn't the above protection, > especially since the patch 2/20 removes q->elevator use in > wbt_enable_default(). > Yes agreed, and as I understand XXX_FLAG_DISABLE_WBT was earlier elevator_queue->flags but now (with patch 2/20) it has been moved to request_queue->flags. As elevator_change_done first puts elevator_queue object which would potentially releases/frees the elevator_queue object. Next while we enable wbt (in elevator_change_done) we may not have access to the elevator_queue object and so now we reference QUEUE_FLAG_DISABLE_WBT using request_queue->flags. That's, I believe, the purpose of patch 2/20. However even with patch 2/20 change, both elevator_change_done and ioc_qos_write or queue_wb_lat_store may run in parallel, isn't it? therad1: blk_mq_update_nr_hw_queues -> __blk_mq_update_nr_hw_queues -> elevator_change_done -> wbt_enable_default -> wbt_init -> wbt_update_limits therad2: queue_wb_lat_store -> wbt_set_min_lat -> wbt_update_limits Thanks, --Nilay