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Date:   Thu, 24 Jun 2021 14:31:22 +0800
From:   Can Guo <cang@codeaurora.org>
To:     Adrian Hunter <adrian.hunter@intel.com>
Cc:     asutoshd@codeaurora.org, nguyenb@codeaurora.org,
        hongwus@codeaurora.org, ziqichen@codeaurora.org,
        linux-scsi@vger.kernel.org, kernel-team@android.com,
        Alim Akhtar <alim.akhtar@samsung.com>,
        Avri Altman <avri.altman@wdc.com>,
        "James E.J. Bottomley" <jejb@linux.ibm.com>,
        "Martin K. Petersen" <martin.petersen@oracle.com>,
        Stanley Chu <stanley.chu@mediatek.com>,
        Bean Huo <beanhuo@micron.com>,
        Jaegeuk Kim <jaegeuk@kernel.org>,
        open list <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v4 06/10] scsi: ufs: Remove host_sem used in
 suspend/resume
In-Reply-To: <a654d2ef-b333-1c56-42c6-3d69e9f44bd0@intel.com>
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On 2021-06-24 14:23, Adrian Hunter wrote:
> On 24/06/21 9:12 am, Can Guo wrote:
>> On 2021-06-24 13:52, Adrian Hunter wrote:
>>> On 24/06/21 5:16 am, Can Guo wrote:
>>>> On 2021-06-23 22:30, Adrian Hunter wrote:
>>>>> On 23/06/21 10:35 am, Can Guo wrote:
>>>>>> To protect system suspend/resume from being disturbed by error 
>>>>>> handling,
>>>>>> instead of using host_sem, let error handler call 
>>>>>> lock_system_sleep() and
>>>>>> unlock_system_sleep() which achieve the same purpose. Remove the 
>>>>>> host_sem
>>>>>> used in suspend/resume paths to make the code more readable.
>>>>>> 
>>>>>> Suggested-by: Bart Van Assche <bvanassche@acm.org>
>>>>>> Signed-off-by: Can Guo <cang@codeaurora.org>
>>>>>> ---
>>>>>>  drivers/scsi/ufs/ufshcd.c | 12 +++++++-----
>>>>>>  1 file changed, 7 insertions(+), 5 deletions(-)
>>>>>> 
>>>>>> diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
>>>>>> index 3695dd2..a09e4a2 100644
>>>>>> --- a/drivers/scsi/ufs/ufshcd.c
>>>>>> +++ b/drivers/scsi/ufs/ufshcd.c
>>>>>> @@ -5907,6 +5907,11 @@ static void 
>>>>>> ufshcd_clk_scaling_suspend(struct ufs_hba *hba, bool suspend)
>>>>>> 
>>>>>>  static void ufshcd_err_handling_prepare(struct ufs_hba *hba)
>>>>>>  {
>>>>>> +    /*
>>>>>> +     * It is not safe to perform error handling while suspend or 
>>>>>> resume is
>>>>>> +     * in progress. Hence the lock_system_sleep() call.
>>>>>> +     */
>>>>>> +    lock_system_sleep();
>>>>> 
>>>>> It looks to me like the system takes this lock quite early, even 
>>>>> before
>>>>> freezing tasks, so if anything needs the error handler to run it 
>>>>> will
>>>>> deadlock.
>>>> 
>>>> Hi Adrian,
>>>> 
>>>> UFS/hba system suspend/resume does not invoke or call error handling 
>>>> in a
>>>> synchronous way. So, whatever UFS errors (which schedules the error 
>>>> handler)
>>>> happens during suspend/resume, error handler will just wait here 
>>>> till system
>>>> suspend/resume release the lock. Hence no worries of deadlock here.
>>> 
>>> It looks to me like the state can change to 
>>> UFSHCD_STATE_EH_SCHEDULED_FATAL
>>> and since user processes are not frozen, nor file systems sync'ed, 
>>> everything
>>> is going to deadlock.
>>> i.e.
>>> I/O is blocked waiting on error handling
>>> error handling is blocked waiting on lock_system_sleep()
>>> suspend is blocked waiting on I/O
>>> 
>> 
>> Hi Adrian,
>> 
>> First of all, enter_state(suspend_state_t state) uses 
>> mutex_trylock(&system_transition_mutex).
> 
> Yes, in the case I am outlining it gets the mutex.
> 
>> Second, even that happens, in ufshcd_queuecommand(), below logic will 
>> break the cycle, by
>> fast failing the PM request (below codes are from the code tip with 
>> this whole series applied).
> 
> It won't get that far because the suspend will be waiting to sync 
> filesystems.
> Filesystems will be waiting on I/O.
> I/O will be waiting on the error handler.
> The error handler will be waiting on system_transition_mutex.
> But system_transition_mutex is already held by PM core.

Hi Adrian,

You are right.... I missed the action of syncing filesystems...

Using back host_sem in suspend_prepare()/resume_complete() won't have 
this
problem of deadlock, right?

Thanks,

Can Guo.

> 
>> 
>>         case UFSHCD_STATE_EH_SCHEDULED_FATAL:
>>                 /*
>>                  * ufshcd_rpm_get_sync() is used at error handling 
>> preparation
>>                  * stage. If a scsi cmd, e.g., the SSU cmd, is sent 
>> from the
>>                  * PM ops, it can never be finished if we let SCSI 
>> layer keep
>>                  * retrying it, which gets err handler stuck forever. 
>> Neither
>>                  * can we let the scsi cmd pass through, because UFS 
>> is in bad
>>                  * state, the scsi cmd may eventually time out, which 
>> will get
>>                  * err handler blocked for too long. So, just fail the 
>> scsi cmd
>>                  * sent from PM ops, err handler can recover PM error 
>> anyways.
>>                  */
>>                 if (cmd->request->rq_flags & RQF_PM) {
>>                         hba->force_reset = true;
>>                         set_host_byte(cmd, DID_BAD_TARGET);
>>                         cmd->scsi_done(cmd);
>>                         goto out;
>>                 }
>>                 fallthrough;
>>         case UFSHCD_STATE_RESET:
>> 
>> Thanks,
>> 
>> Can Guo.
>> 
>>>> 
>>>> Thanks,
>>>> 
>>>> Can Guo.
>>>> 
>>>>> 
>>>>>>      ufshcd_rpm_get_sync(hba);
>>>>>>      if 
>>>>>> (pm_runtime_status_suspended(&hba->sdev_ufs_device->sdev_gendev) 
>>>>>> ||
>>>>>>          hba->is_wlu_sys_suspended) {
>>>>>> @@ -5951,6 +5956,7 @@ static void 
>>>>>> ufshcd_err_handling_unprepare(struct ufs_hba *hba)
>>>>>>          ufshcd_clk_scaling_suspend(hba, false);
>>>>>>      ufshcd_clear_ua_wluns(hba);
>>>>>>      ufshcd_rpm_put(hba);
>>>>>> +    unlock_system_sleep();
>>>>>>  }
>>>>>> 
>>>>>>  static inline bool ufshcd_err_handling_should_stop(struct ufs_hba 
>>>>>> *hba)
>>>>>> @@ -9053,16 +9059,13 @@ static int ufshcd_wl_suspend(struct device 
>>>>>> *dev)
>>>>>>      ktime_t start = ktime_get();
>>>>>> 
>>>>>>      hba = shost_priv(sdev->host);
>>>>>> -    down(&hba->host_sem);
>>>>>> 
>>>>>>      if (pm_runtime_suspended(dev))
>>>>>>          goto out;
>>>>>> 
>>>>>>      ret = __ufshcd_wl_suspend(hba, UFS_SYSTEM_PM);
>>>>>> -    if (ret) {
>>>>>> +    if (ret)
>>>>>>          dev_err(&sdev->sdev_gendev, "%s failed: %d\n", __func__,  
>>>>>> ret);
>>>>>> -        up(&hba->host_sem);
>>>>>> -    }
>>>>>> 
>>>>>>  out:
>>>>>>      if (!ret)
>>>>>> @@ -9095,7 +9098,6 @@ static int ufshcd_wl_resume(struct device 
>>>>>> *dev)
>>>>>>          hba->curr_dev_pwr_mode, hba->uic_link_state);
>>>>>>      if (!ret)
>>>>>>          hba->is_wlu_sys_suspended = false;
>>>>>> -    up(&hba->host_sem);
>>>>>>      return ret;
>>>>>>  }
>>>>>>  #endif
>>>>>>