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Subject: Re: [PATCH v4 1/8] scsi: ufs: Flush exception event before suspend
To:     Can Guo <cang@codeaurora.org>
Cc:     asutoshd@codeaurora.org, nguyenb@codeaurora.org,
        hongwus@codeaurora.org, rnayak@codeaurora.org,
        linux-scsi@vger.kernel.org, kernel-team@android.com,
        saravanak@google.com, salyzyn@google.com,
        Sayali Lokhande <sayalil@codeaurora.org>,
        Alim Akhtar <alim.akhtar@samsung.com>,
        Avri Altman <avri.altman@wdc.com>,
        Pedro Sousa <pedrom.sousa@synopsys.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>,
        Venkat Gopalakrishnan <venkatg@codeaurora.org>,
        Tomas Winkler <tomas.winkler@intel.com>,
        open list <linux-kernel@vger.kernel.org>
References: <1579764349-15578-1-git-send-email-cang@codeaurora.org>
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From:   Bart Van Assche <bvanassche@acm.org>
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Date:   Mon, 3 Feb 2020 19:12:55 -0800
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On 2020-02-02 22:23, Can Guo wrote:
> On 2020-01-26 11:29, Bart Van Assche wrote:
>> On 2020-01-22 23:25, Can Guo wrote:
>>>              break;
>>>          case UPIU_TRANSACTION_REJECT_UPIU:
>>>              /* TODO: handle Reject UPIU Response */
>>> @@ -5215,7 +5222,14 @@ static void
>>> ufshcd_exception_event_handler(struct work_struct *work)
>>>
>>>  out:
>>>      scsi_unblock_requests(hba->host);
>>> -    pm_runtime_put_sync(hba->dev);
>>> +    /*
>>> +     * pm_runtime_get_noresume is called while scheduling
>>> +     * eeh_work to avoid suspend racing with exception work.
>>> +     * Hence decrement usage counter using pm_runtime_put_noidle
>>> +     * to allow suspend on completion of exception event handler.
>>> +     */
>>> +    pm_runtime_put_noidle(hba->dev);
>>> +    pm_runtime_put(hba->dev);
>>>      return;
>>>  }
>>>
>>> @@ -7901,6 +7915,7 @@ static int ufshcd_suspend(struct ufs_hba *hba,
>>> enum ufs_pm_op pm_op)
>>>              goto enable_gating;
>>>      }
>>>
>>> +    flush_work(&hba->eeh_work);
>>>      ret = ufshcd_link_state_transition(hba, req_link_state, 1);
>>>      if (ret)
>>>          goto set_dev_active;
>>
>> I think this patch introduces a new race condition, namely the following:
>> - ufshcd_slave_destroy() tests pm_op_in_progress and reads the value
>>   zero from that variable.
>> - ufshcd_suspend() sets hba->pm_op_in_progress to one.
>> - ufshcd_slave_destroy() calls schedule_work().
>>
>> How about fixing this race condition by calling
>> pm_runtime_get_noresume() before checking pm_op_in_progress and by
>> reallowing resume if no work is scheduled?
> 
> If you apply this patch, you will find the change is not in
> ufshcd_slave_destroy(), but in ufshcd_transfer_rsp_status().
> So the racing you mentioned above does not exist.

Hi Can,

Apparently I got a function name wrong. Can the following race condition
happen:
- ufshcd_transfer_rsp_status() tests pm_op_in_progress and reads the
  value zero from that variable.
- ufshcd_suspend() sets hba->pm_op_in_progress to one.
- ufshcd_suspend() calls flush_work(&hba->eeh_work).
- ufshcd_transfer_rsp_status() calls schedule_work(&hba->eeh_work).

Thanks,

Bart.