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Date:   Mon, 27 Jul 2020 15:30:10 +0800
From:   Can Guo <cang@codeaurora.org>
To:     Stanley Chu <stanley.chu@mediatek.com>
Cc:     linux-scsi@vger.kernel.org, martin.petersen@oracle.com,
        avri.altman@wdc.com, alim.akhtar@samsung.com, jejb@linux.ibm.com,
        bvanassche@acm.org, beanhuo@micron.com, asutoshd@codeaurora.org,
        matthias.bgg@gmail.com, linux-mediatek@lists.infradead.org,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        kuohong.wang@mediatek.com, peter.wang@mediatek.com,
        chun-hung.wu@mediatek.com, andy.teng@mediatek.com,
        chaotian.jing@mediatek.com, cc.chou@mediatek.com
Subject: Re: [PATCH v4] scsi: ufs: Quiesce all scsi devices before shutdown
In-Reply-To: <20200724140140.18186-1-stanley.chu@mediatek.com>
References: <20200724140140.18186-1-stanley.chu@mediatek.com>
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Hi Stanley,

On 2020-07-24 22:01, 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, specifically quiesce all SCSI devices
> before UFS shutdown to block all I/O request sending from block
> layer.
> 
> 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)
> 

I don't quite get it, how can you prevent block layer PM from iniating
hba runtime resume by quiescing the scsi devices? Block layer PM
iniates hba async runtime resume in blk_queue_enter(). But quiescing
the scsi devices can only prevent general I/O requests from passing
through scsi_queue_rq() callback.

Say hba is runtime suspended, if an I/O request to sda is sent from
block layer (sda must be runtime suspended as well at this time),
blk_queue_enter() initiates async runtime resume for sda. But since
sda's parents are also runtime suspended, the RPM framework shall do
runtime resume to the devices in the sequence hba->host->target->sda.
In this case, ufshcd_resume() still runs concurrently, no?

Thanks,

Can Guo.

> 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>
> ---
>  drivers/scsi/ufs/ufshcd.c | 29 +++++++++++++++++++++++++++++
>  1 file changed, 29 insertions(+)
> 
> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
> index 9d180da77488..2e18596f3a8e 100644
> --- a/drivers/scsi/ufs/ufshcd.c
> +++ b/drivers/scsi/ufs/ufshcd.c
> @@ -159,6 +159,12 @@ struct ufs_pm_lvl_states ufs_pm_lvl_states[] = {
>  	{UFS_POWERDOWN_PWR_MODE, UIC_LINK_OFF_STATE},
>  };
> 
> +#define ufshcd_scsi_for_each_sdev(fn) \
> +	list_for_each_entry(starget, &hba->host->__targets, siblings) { \
> +		__starget_for_each_device(starget, NULL, \
> +					  fn); \
> +	}
> +
>  static inline enum ufs_dev_pwr_mode
>  ufs_get_pm_lvl_to_dev_pwr_mode(enum ufs_pm_level lvl)
>  {
> @@ -8620,6 +8626,13 @@ int ufshcd_runtime_idle(struct ufs_hba *hba)
>  }
>  EXPORT_SYMBOL(ufshcd_runtime_idle);
> 
> +static void ufshcd_quiesce_sdev(struct scsi_device *sdev, void *data)
> +{
> +	/* Suspended devices are already quiesced so can be skipped */
> +	if (!pm_runtime_suspended(&sdev->sdev_gendev))
> +		scsi_device_quiesce(sdev);
> +}
> +
>  /**
>   * ufshcd_shutdown - shutdown routine
>   * @hba: per adapter instance
> @@ -8631,6 +8644,7 @@ 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;
> @@ -8644,6 +8658,21 @@ int ufshcd_shutdown(struct ufs_hba *hba)
>  			goto out;
>  	}
> 
> +	/*
> +	 * 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.
> +	 */
> +	ufshcd_scsi_for_each_sdev(ufshcd_quiesce_sdev);
> +
>  	ret = ufshcd_suspend(hba, UFS_SHUTDOWN_PM);
>  out:
>  	if (ret)