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Message-ID: <f1c997f3-66e4-3f1f-08f5-83449b65c397@intel.com>
Date: Thu, 24 Jun 2021 13:04:16 +0300
From: Adrian Hunter <adrian.hunter@...el.com>
To: Can Guo <cang@...eaurora.org>
Cc: asutoshd@...eaurora.org, nguyenb@...eaurora.org,
hongwus@...eaurora.org, ziqichen@...eaurora.org,
linux-scsi@...r.kernel.org, kernel-team@...roid.com,
Alim Akhtar <alim.akhtar@...sung.com>,
Avri Altman <avri.altman@....com>,
"James E.J. Bottomley" <jejb@...ux.ibm.com>,
"Martin K. Petersen" <martin.petersen@...cle.com>,
Stanley Chu <stanley.chu@...iatek.com>,
Bean Huo <beanhuo@...ron.com>,
Jaegeuk Kim <jaegeuk@...nel.org>,
open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 06/10] scsi: ufs: Remove host_sem used in
suspend/resume
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@....org>
>>>>>>> Signed-off-by: Can Guo <cang@...eaurora.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
>>>>>>>
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