Re: [PATCH v7] scsi: ufs: Quiesce all scsi devices before shutdown

Can Guo
2020-08-03T12:05:07Z

Slightly updated my comments

On 2020-08-03 19:50, Can Guo wrote:
> Hi Stanley,
>
> On 2020-08-03 18:04, Stanley Chu wrote:
>> Currently I/O request could be still submitted to UFS device while
>> UFS is working on shutdown flow. This may lead to racing as below
>> scenarios and finally system may crash due to unclocked register
>> accesses.
>>
>> To fix this kind of issues, in ufshcd_shutdown(),
>>
>> 1. Use pm_runtime_get_sync() instead of resuming UFS device by
>> ufshcd_runtime_resume() "internally" to let runtime PM framework
>> manage and prevent concurrent runtime operations by incoming I/O
>> requests.
>>
>> 2. Specifically quiesce all SCSI devices to block all I/O requests
>> after device is resumed.
>>
>> Example of racing scenario: While UFS device is runtime-suspended
>>
>> Thread #1: Executing UFS shutdown flow, e.g.,
>> ufshcd_suspend(UFS_SHUTDOWN_PM)
>>
>> Thread #2: Executing runtime resume flow triggered by I/O request,
>> e.g., ufshcd_resume(UFS_RUNTIME_PM)
>>
>> This breaks the assumption that UFS PM flows can not be running
>> concurrently and some unexpected racing behavior may happen.
>>
>> Signed-off-by: Stanley Chu <stanley.chu@mediatek.com>
>> ---
>> Changes:
>> - Since v6:
>> - Do quiesce to all SCSI devices.
>> - Since v4:
>> - Use pm_runtime_get_sync() instead of resuming UFS device by
>> ufshcd_runtime_resume() "internally".
>> ---
>> drivers/scsi/ufs/ufshcd.c | 27 ++++++++++++++++++++++-----
>> 1 file changed, 22 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
>> index 307622284239..7cb220b3fde0 100644
>> --- a/drivers/scsi/ufs/ufshcd.c
>> +++ b/drivers/scsi/ufs/ufshcd.c
>> @@ -8640,6 +8640,7 @@ EXPORT_SYMBOL(ufshcd_runtime_idle);
>> int ufshcd_shutdown(struct ufs_hba *hba)
>> {
>> int ret = 0;
>> + struct scsi_target *starget;
>>
>> if (!hba->is_powered)
>> goto out;
>> @@ -8647,11 +8648,27 @@ int ufshcd_shutdown(struct ufs_hba *hba)
>> if (ufshcd_is_ufs_dev_poweroff(hba) && ufshcd_is_link_off(hba))
>> goto out;
>>
>> - if (pm_runtime_suspended(hba->dev)) {
>> - ret = ufshcd_runtime_resume(hba);
>> - if (ret)
>> - goto out;
>> - }
>> + /*
>> + * Let runtime PM framework manage and prevent concurrent runtime
>> + * operations with shutdown flow.
>> + */
>> + pm_runtime_get_sync(hba->dev);
>> +
>> + /*
>> + * Quiesce all SCSI devices to prevent any non-PM requests sending
>> + * from block layer during and after shutdown.
>> + *
>> + * Here we can not use blk_cleanup_queue() since PM requests
>> + * (with BLK_MQ_REQ_PREEMPT flag) are still required to be sent
>> + * through block layer. Therefore SCSI command queued after the
>> + * scsi_target_quiesce() call returned will block until
>> + * blk_cleanup_queue() is called.
>> + *
>> + * Besides, scsi_target_"un"quiesce (e.g., scsi_target_resume) can
>> + * be ignored since shutdown is one-way flow.
>> + */
>> + list_for_each_entry(starget, &hba->host->__targets, siblings)
>> + scsi_target_quiesce(starget);
>>
>
> Sorry for misleading you to scsi_target_quiesce(), maybe below is
> better.
>
> shost_for_each_device(sdev, hba->host)
> scsi_device_quiesce(sdev);
>
> We may need to discuss more about this quiesce part since I missed
> something.
>
> After we quiesce the scsi devices, only PM requests are allowed, but it
> is still not safe: [1] PM requests can still pass through, [2] there
> can
> be tasks/reqs present in doorbells before the devices are quiesced. So,
> these tasks/reqs in [1] and [2] can still be flying in parallel while
> ufshcd_suspend is running.
>
> How about only quiescing the UFS device well known scsi device but
> using
> freeze_queue to the other scsi devices? blk_mq_freeze_queue can
> eliminate
> the risks mentioned in [1] and [2].
>
> shost_for_each_device(sdev, hba->host) {
> if (sdev == hba->sdev_ufs_device)
> scsi_device_quiesce(sdev);
> else
> blk_mq_freeze_queue(sdev->request_queue);
> }
>
> IF blk_mq_freeze_queue is not allowed to be used by LLD (I think we can
> use it as I recalled Bart used to use it in one of his changes to UFS
> scaling),
> we can use scsi_remove_device instead, it changes scsi device's state
> to
> SDEV_DEL and calls blk_cleanup_queue.
>
> We can also make changes like below. [1] is to make sure no more PM
> requests
> sent to scsi devices, [2] is make sure doorbells are cleared before
> invoke
> ufshcd_suspend.
>
> shost_for_each_device(sdev, hba->host) {
> scsi_autopm_get_device(sdev); [1]
> scsi_device_quiesce(sdev);
> }
>
> ufshcd_wait_for_doorbell_clr(hba, U64_MAX); [2]
>
> Please let me know which one you prefer or if you have better idea,
> thanks!
>
> Regards,
>
> Can Guo.
>
>> ret = ufshcd_suspend(hba, UFS_SHUTDOWN_PM);