linux-kernel - Re: [PATCH v3 8/9] scsi: ufs: Update the fast abort path in ufshcd

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Date:   Mon, 14 Jun 2021 11:49:10 -0700
From:   Bart Van Assche <bvanassche@....org>
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 v3 8/9] scsi: ufs: Update the fast abort path in
 ufshcd_abort() for PM requests

On 6/13/21 7:42 AM, Can Guo wrote:
> 2. ufshcd_abort() invokes ufshcd_err_handler() synchronously can have a
> live lock issue, which is why I chose the asynchronous way (from the first
> day I started to fix error handling). The live lock happens when abort
> happens
> to a PM request, e.g., a SSU cmd sent from suspend/resume. Because UFS
> error
> handler is synchronized with suspend/resume (by calling
> pm_runtime_get_sync()
> and lock_system_sleep()), the sequence is like:
> [1] ufshcd_wl_resume() sends SSU cmd
> [2] ufshcd_abort() calls UFS error handler
> [3] UFS error handler calls lock_system_sleep() and pm_runtime_get_sync()
> 
> In above sequence, either lock_system_sleep() or pm_runtime_get_sync()
> shall
> be blocked - [3] is blocked by [1], [2] is blocked by [3], while [1] is
> blocked by [2].
> 
> For PM requests, I chose to abort them fast to unblock suspend/resume,
> suspend/resume shall fail of course, but UFS error handler recovers
> PM errors anyways.

In the above sequence, does [2] perhaps refer to aborting the SSU
command submitted in step [1] (this is not clear to me)? If so, how
about breaking the circular waiting cycle as follows:
- If it can happen that SSU succeeds after more than scsi_timeout
  seconds, define a custom timeout handler. From inside the timeout
  handler, schedule a link check and return BLK_EH_RESET_TIMER. If the
  link is no longer operational, run the error handler. If the link
  cannot be recovered by the error handler, fail all pending commands.
  This will prevent that ufshcd_abort() is called if a SSU command takes
  longer than expected. See also commit 0dd0dec1677e.
- Modify the UFS error handler such that it accepts a context argument.
  The context argument specifies whether or not the UFS error handler is
  called from inside a system suspend or system resume handler. If the
  UFS error handler is called from inside a system suspend or resume
  callback, skip the lock_system_sleep() and unlock_system_sleep()
  calls.

Thanks,

Bart.