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Subject: Re: [PATCH v4 06/10] scsi: ufs: Remove host_sem used in
 suspend/resume
To:     Can Guo <cang@codeaurora.org>
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>
References: <1624433711-9339-1-git-send-email-cang@codeaurora.org>
 <1624433711-9339-8-git-send-email-cang@codeaurora.org>
 <ed59d61a-6951-2acd-4f89-40f8dc5015e1@intel.com>
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 <a87e5ca5-390f-8ca0-41bf-27cdc70e3316@intel.com>
 <1b351766a6e40d0df90b3adec964eb33@codeaurora.org>
 <a654d2ef-b333-1c56-42c6-3d69e9f44bd0@intel.com>
 <3970b015e444c1f1714c7e7bd4c44651@codeaurora.org>
From:   Adrian Hunter <adrian.hunter@intel.com>
Organization: Intel Finland Oy, Registered Address: PL 281, 00181 Helsinki,
 Business Identity Code: 0357606 - 4, Domiciled in Helsinki
Message-ID: <f1c997f3-66e4-3f1f-08f5-83449b65c397@intel.com>
Date:   Thu, 24 Jun 2021 13:04:16 +0300
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On 24/06/21 9:31 am, Can Guo wrote:
> 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?

I am not sure, but what was problem that the V3 patch was fixing?
Can you give an example?

> 
> 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
>>>>>>>