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

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Message-ID: <75527f0ba5d315d6edbf800a2ddcf8c7@codeaurora.org>
Date:   Sun, 13 Jun 2021 22:42:55 +0800
From:   Can Guo <cang@...eaurora.org>
To:     Bart Van Assche <bvanassche@....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

Hi Bart,

On 2021-06-13 00:50, Bart Van Assche wrote:
> On 6/12/21 12:07 AM, Can Guo wrote:
>> Sigh... I also want my life and work to be easier...
> 
> How about reducing the number of states and state transitions in the 
> UFS
> driver? One source of complexity is that ufshcd_err_handler() is 
> scheduled
> independently of the SCSI error handler and hence may run concurrently
> with the SCSI error handler. Has the following already been considered?
> - Call ufshcd_err_handler() synchronously from ufshcd_abort() and
> ufshcd_eh_host_reset_handler() instead of asynchronously.

1. ufshcd_eh_host_reset_handler() invokes ufshcd_err_handler() and 
flushes
it, so it is synchronous. ufshcd_eh_host_reset_handler() used to call
reset_and_restore() directly, which can run concurrently with UFS error 
handler,
so I fixed it last year [1].

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.

> - Call scsi_schedule_eh() from ufshcd_uic_pwr_ctrl() and
> ufshcd_check_errors() instead of ufshcd_schedule_eh_work().

When ufshcd_uic_pwr_ctrl() and/or ufshcd_check_errors() report errors,
usually they are fatal errors, according to UFSHCI spec, SW should 
re-probe
UFS to recover.

However scsi_schedule_eh() does more than that - scsi_unjam_host() sends
request sense cmd and calls scsi_eh_ready_devs(), while 
scsi_eh_ready_devs()
sends test unit ready cmd and calls all the way down to 
scsi_eh_device/target/
bus/host_reset(). But we only need scsi_eh_host_reset() in this case. I 
know
you have concerns that scsi_schedule_eh() may run concurrently with UFS 
error
handler, but as I mentioned above in [1] - I've made 
ufshcd_eh_host_reset_handler()
synchronized with UFS error handler, hope that can ease your concern.

I am not saying your idea won't work, it is a good suggestion. I will 
try
it after these changes go in, because it would require extra effort and 
the
effort won't be minor - I need to consider how to remove/reduce the 
ufshcd
states along with the change and the error injection and stability test 
all
over again, which is a long way to go. As for now, at least current 
changes
works well as per my test and we really need these changes for 
Andriod12-5.10.

Thanks,

Can Guo.

> 
> These changes will guarantee that all commands have completed or timed
> out before ufshcd_err_handler() is called. I think that would allow to
> remove e.g. the following code from the error handler:
> 
> 	ufshcd_scsi_block_requests(hba);
> 	/* Drain ufshcd_queuecommand() */
> 	down_write(&hba->clk_scaling_lock);
> 	up_write(&hba->clk_scaling_lock);
> 
> Thanks,
> 
> Bart.