Re: [PATCH v1 1/2] scsi: ufs: Fix unbalanced scsi_block_reqs_cnt caused by ufshcd_hold()

[Can Guo]

On 2020-11-03 15:07, Stanley Chu wrote:
> Hi Can,
>
> On Mon, 2020-11-02 at 22:24 -0800, Can Guo wrote:
> > The scsi_block_reqs_cnt increased in ufshcd_hold() is supposed to be
> > decreased back in ufshcd_ungate_work() in a paired way. However, if
> > specific ufshcd_hold/release sequences are met, it is possible that
> > scsi_block_reqs_cnt is increased twice but only one ungate work is
> > queued. To make sure scsi_block_reqs_cnt is handled by ufshcd_hold() 
> > and
>
> Just curious that how could this be possible? Would you have some 
> failed
> examples?

[1] One gate_work() is in the workqueue, not yet executed, now clk state 
== REQ_CLKS_OFF.
[2] ufshcd_queuecommand() calls ufshcd_hold(async == ture) -> 
active_req++ -> scsi_block_reqs_cnt++ -> REQ_CLKS_ON -> queue ungate 
work -> active_req-- -> return -EAGAIN.
[3] Now gate_work() starts to run, but since the clk state is 
REQ_CLKS_ON, gate_work() just sets clk state to CLKS_ON and bail.
[3] Someone calls ufshcd_hold(async == false) -> do something -> 
ufshcd_release() -> clk state is changed to REQ_CLKS_OFF. Note that, 
till now, ungate_work() is still in the work queue, not executed yet.
[4] Now, if someone calls ufshcd_hold(), we will hit the issue.

Above sequence is a very common clk gate/ungate sequence. The issue
is because ungate_work is queued but cannot be executed in time. In my
case, I see the ungate_work is somehow delayed for about 150ms. This
change has been tested by customers on multiple platforms. And you
can tell from the code that it won't break anything. :)

Thanks,

Can Guo.

> > ufshcd_ungate_work() in a paired way, increase it only if queue_work()
> > returns true.
> >
> > Signed-off-by: Can Guo <cang@xxxxxxxxxxxxxx>
> > Reviewed-by: Hongwu Su <hongwus@xxxxxxxxxxxxxx>
> > ---
> >  drivers/scsi/ufs/ufshcd.c | 6 +++---
> >  1 file changed, 3 insertions(+), 3 deletions(-)
> >
> > diff --git a/drivers/scsi/ufs/ufshcd.c b/drivers/scsi/ufs/ufshcd.c
> > index 847f355..efa7d86 100644
> > --- a/drivers/scsi/ufs/ufshcd.c
> > +++ b/drivers/scsi/ufs/ufshcd.c
> > @@ -1634,12 +1634,12 @@ int ufshcd_hold(struct ufs_hba *hba, bool 
> > async)
> >  		 */
> >  		/* fallthrough */
> >  	case CLKS_OFF:
> > -		ufshcd_scsi_block_requests(hba);
> >  		hba->clk_gating.state = REQ_CLKS_ON;
> >  		trace_ufshcd_clk_gating(dev_name(hba->dev),
> >  					hba->clk_gating.state);
> > -		queue_work(hba->clk_gating.clk_gating_workq,
> > -			   &hba->clk_gating.ungate_work);
> > +		if (queue_work(hba->clk_gating.clk_gating_workq,
> > +			       &hba->clk_gating.ungate_work))
> > +			ufshcd_scsi_block_requests(hba);
> >  		/*
> >  		 * fall through to check if we should wait for this
> >  		 * work to be done or not.
>
> Thanks,
> Stanley Chu