linux-kernel - Re: [PATCH v2 5/6] scsi: ufs: Let host_sem cover the entire system suspend/resume

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Date:   Tue, 25 May 2021 10:04:23 +0800
From:   Can Guo <cang@...eaurora.org>
To:     Bart Van Assche <bvanassche@....org>
Cc:     asutoshd@...eaurora.org, nguyenb@...eaurora.org,
        hongwus@...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 v2 5/6] scsi: ufs: Let host_sem cover the entire system
 suspend/resume

Hi Bart,

On 2021-05-25 00:56, Bart Van Assche wrote:
> On 5/24/21 1:47 AM, Can Guo wrote:
>> UFS error handling now is doing more than just re-probing, but also 
>> sending
>> scsi cmds, e.g., for clearing UACs, and recovering runtime PM error, 
>> which
>> may change runtime status of scsi devices. To protect system 
>> suspend/resume
>> from being disturbed by error handling, move the host_sem from wl pm 
>> ops
>> to ufshcd_suspend_prepare() and ufshcd_resume_complete().
> 
> Other SCSI LLDs can perform error handling while system suspend/resume
> is in progress. Why can't the UFS driver do this?

I don't know about other SCSI LLDs, but UFS error handling is basically
doing a re-probe/re-initialization to UFS device. Having UFS error 
handling
running in parallel with system suspend/resume, neither of them will end
up well.

I didn't design all this, it is just happening, I am trying to fix it 
and
semaphore works well for me. I am really glad to see someone cares about
error handling and fix it with better ideas (maybe using WQ_FREEZABLE) 
later.

> 
> Additionally, please document what the purpose of host_sem is before
> making any changes to how host_sem is used. The only documentation I
> have found of host_sem is the following: "* @host_sem: semaphore used 
> to
> serialize concurrent contexts". To me that text is less than useful
> since semaphores are almost always used to serialize concurrent code.
> 

Sure, host_sem is actually preventing cocurrency happens among any of
contexts, such as sysfs access, shutdown, error handling, system
suspend/resume and async probe, I will update its message in next 
version.

Thanks,

Can Guo.

> Thanks,
> 
> Bart.