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

Can Guo
2020-08-03T12:53:52Z

Hi Stanley,

Sorry for the noises, please ignore my previous 2 mails and let's
focus on this one.

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 - PM requests can still pass through.

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 risk.

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 use scsi_autopm_get_device like below. It is to make sure
no more PM requests sent to scsi devices (since PM requests are only
sent
during PM ops).

shost_for_each_device(sdev, hba->host) {
scsi_autopm_get_device(sdev);
scsi_device_quiesce(sdev);
}

Please let me know which one do you prefer or if you have better ideas,
thanks!

Regards,

Can Guo.

> ret = ufshcd_suspend(hba, UFS_SHUTDOWN_PM);