Re: [PATCH v2] scsi: core: Fix hang of freezing queue between blocking and running device

[Li Jinlin <lijinlin3@huawei.com>]

On 2021/8/17 16:46, Li Jinlin wrote:
> On 2021/8/9 22:13, Li Jinlin wrote:
>> From: Li Jinlin <lijinlin3@huawei.com>
>>
>> We found a hang issue, the test steps are as follows:
>>   1. blocking device via scsi_device_set_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
>>
>> Step 3 and 4 should finish this work after step 4, but they hangs.
>>
>>   CPU#0               CPU#1                CPU#2
>>   ---------------     ----------------     ----------------
>>                                            Step 1: blocking device
>>
>>                                            Step 2: dd xxxx
>>                                                   ^^^^^^ get request
>>                                                          q_usage_counter++
>>
>>                       Step 3: 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
>>
>>   Step 4: 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--
>>
>> Step 3 and 4 wait for each other.
>>
>> To fix this, we need to run queue before rescanning device when the device
>> state 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>
>> ---
>> changes since v1 send with Message-ID:
>> 20210805143231.1713299-1-lijinlin3@huawei.com
>>
>>  - Modify the subject to make it distinct
>>  - Modify the message to fix typo and make it distinct
>>  - Reduce the number of SOB
>>
>>  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.
>>  	 */
>>  	if (ret == 0 && state == SDEV_RUNNING) {
>> -		scsi_rescan_device(dev);
>>  		blk_mq_run_hw_queues(sdev->request_queue, true);
>> +		scsi_rescan_device(dev);
>>  	}
>>  	mutex_unlock(&sdev->state_mutex);
>>  
>>
> 
> Ping.
> 
> Thanks,
> Li Jinlin
> 
Hello Guys,

Ping again.

Otherwise, I think that rescan device is not necessary when the device
state changes from blocked to running. Run queue is not necessary either
when the device state changes from offline to running.

I want to add judgment of the old device state before rescanning device
or running queue.

I'd like to know your opinions.

Thanks,
Li Jinlin