Re: [PATCH v2 2/4] scsi: ufs: Fix imbalanced scsi_block_reqs_cnt caused by ufshcd_hold()

[Can Guo]

On 2020-07-14 11:41, Bart Van Assche wrote:
> On 2020-07-13 19:28, 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
> > ufshcd_ungate_work() in a paired way, increase it only if queue_work()
> > returns true.
> >
> > Signed-off-by: Can Guo <cang@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 ebf7a95..33214bb 100644
> > --- a/drivers/scsi/ufs/ufshcd.c
> > +++ b/drivers/scsi/ufs/ufshcd.c
> > @@ -1611,12 +1611,12 @@ 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.
>
> Since "ungate_work" involves calling ufshcd_scsi_unblock_requests() and
> since this patch changes the order in which 
> ufshcd_scsi_block_requests()
> and queue_work() are called, I think this patch introduces a race
> condition. Has it been considered to leave the 
> ufshcd_scsi_block_requests()
> call where it is and to call ufshcd_scsi_unblock_requests() if
> queue_work() fails?
>
> Thanks,
>
> Bart.

Hi Bart,

The racing does not exist due to we still hold the spin lock here. 
Before
release the spin lock, the ungate_work, even it starts to run, cannot
move forward as it needs to require the spin lock once in the entrance.

static void ufshcd_ungate_work(struct work_struct *work)
{
...
        spin_lock_irqsave(hba->host->host_lock, flags);
        if (hba->clk_gating.state == CLKS_ON) {
                spin_unlock_irqrestore(hba->host->host_lock, flags);
                goto unblock_reqs;
        }
...
}