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Subject: Re: [PATCH] scsi: core: Run queue first after running device.
To:     <lijinlin3@huawei.com>, <jejb@linux.ibm.com>,
        <martin.petersen@oracle.com>, <linux-scsi@vger.kernel.org>,
        <linux-kernel@vger.kernel.org>
CC:     <bvanassche@acm.org>, <qiulaibin@huawei.com>,
        <linfeilong@huawei.com>, <wubo40@huawei.com>
References: <20210805143231.1713299-1-lijinlin3@huawei.com>
From:   John Garry <john.garry@huawei.com>
Message-ID: <908bb2bb-c511-06a4-e0b6-577d90bb9b57@huawei.com>
Date:   Fri, 6 Aug 2021 09:58:29 +0100
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On 05/08/2021 15:32, lijinlin3@huawei.com wrote:
> From: Li Jinlin<lijinlin3@huawei.com>
> 
> We found a hang issue, the test steps are as follows:
>    1. echo "blocked" >/sys/block/sda/device/state
>    2. dd if=/dev/sda of=/mnt/t.log bs=1M count=10
>    3. echo none > /sys/block/sda/queue/scheduler
>    4. echo "running" >/sys/block/sda/device/state
> 
> Step3 and Step4 should finish this work after Step4, but them hangs.
> 
>    CPU#0               CPU#1                CPU#2
>    ---------------     ----------------     ----------------
>                                             Step1: blocking device
> 
>                                             Step2: dd xxxx
>                                                    ^^^^^^ get request
>                                                           q_usage_counter++
> 
>                        Step3: switching scheculer
>                        elv_iosched_store
>                          elevator_switch
>                            blk_mq_freeze_queue
>                              blk_freeze_queue
>                                > blk_freeze_queue_start
>                                  ^^^^^^ mq_freeze_depth++
> 
>                                > blk_mq_run_hw_queues
>                                  ^^^^^^ can't run queue when dev blocked
> 
>                                > blk_mq_freeze_queue_wait
>                                  ^^^^^^ Hang here!!!
>                                         wait q_usage_counter==0
> 
>    Step4: running device
>    store_state_field
>      scsi_rescan_device
>        scsi_attach_vpd
>          scsi_vpd_inquiry
>            __scsi_execute
>              blk_get_request
>                blk_mq_alloc_request
>                  blk_queue_enter
>                  ^^^^^^ Hang here!!!
>                         wait mq_freeze_depth==0
> 
>      blk_mq_run_hw_queues
>      ^^^^^^ dispatch IO, q_usage_counter will reduce to zero
> 
>                              blk_mq_unfreeze_queue
>                              ^^^^^ mq_freeze_depth--
> 
> Step3 and Step4 wait for each other, caused hangs.
> 
> This requires run queue frist to fix this issue when the device state

frist ?

> changes to SDEV_RUNNING.
> 
> Fixes: f0f82e2476f6 ("scsi: core: Fix capacity set to zero after offlinining device")
> Signed-off-by: Li Jinlin<lijinlin3@huawei.com>
> Signed-off-by: Qiu Laibin<qiulaibin@huawei.com>
> Signed-off-by: Wu Bo<wubo40@huawei.com>

what kind of SoB is this?

> ---
>   drivers/scsi/scsi_sysfs.c | 6 +++---
>   1 file changed, 3 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/scsi/scsi_sysfs.c b/drivers/scsi/scsi_sysfs.c
> index c3a710bceba0..aa701582c950 100644
> --- a/drivers/scsi/scsi_sysfs.c
> +++ b/drivers/scsi/scsi_sysfs.c
> @@ -809,12 +809,12 @@ store_state_field(struct device *dev, struct device_attribute *attr,
>   	ret = scsi_device_set_state(sdev, state);
>   	/*
>   	 * If the device state changes to SDEV_RUNNING, we need to
> -	 * rescan the device to revalidate it, and run the queue to
> -	 * avoid I/O hang.
> +	 * run the queue to avoid I/O hang, and rescan the device
> +	 * to revalidate it.

A bit more description of the IO hang would be useful

>   	 */
>   	if (ret == 0 && state == SDEV_RUNNING) {
> -		scsi_rescan_device(dev);
>   		blk_mq_run_hw_queues(sdev->request_queue, true);
> +		scsi_rescan_device(dev);

This would not have happened if scsi_rescan_device() was ran outside the 
mutex lock region, like I suggested originally.

Indeed, I doubt blk_mq_run_hw_queues() needs to be run with the sdev 
state_mutex held either.

>   	}
>   	mutex_unlock(&sdev->state_mutex);
>   
> --