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

[Can Guo]

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.