Re: [PATCH] scsi: scsi_host_queue_ready: increase busy count early

[Martin Wilck <mwilck@suse.com>]

On Fri, 2021-01-22 at 11:23 +0800, Ming Lei wrote:
> > 
> > diff --git a/drivers/scsi/hosts.c b/drivers/scsi/hosts.c
> > index 2f162603876f..1c452a1c18fd 100644
> > --- a/drivers/scsi/hosts.c
> > +++ b/drivers/scsi/hosts.c
> > @@ -564,6 +564,8 @@ static bool scsi_host_check_in_flight(struct
> > request *rq, void *data,
> >         int *count = data;
> >         struct scsi_cmnd *cmd = blk_mq_rq_to_pdu(rq);
> >  
> > +       /* This pairs with set_bit() in scsi_host_queue_ready() */
> > +       smp_mb__before_atomic();
> 
> So the above barrier orders atomic_read(&shost->host_blocked) and
> test_bit()?

Yes, I believe the change to SCMD_STATE_INFLIGHT should be visible
before host_blocked is tested. For that, I would need to insert a full
smb_mb() instead of smp_mb_after_atomic() below, though ...
right? Which means that the smp_mb__before_atomic() above could be
removed. The important thing is that if two threads execute
scsi_host_queue_ready() simultaneously, one of them must see the
SCMD_STATE_INFLIGHT bit of the other.

Btw, I realized that calling scsi_host_busy() in
scsi_host_queue_ready() is inefficient, because all we need to know
there is whether the value is > 1; we don't need to iterate through the
entire tag space.

Thanks,
Martin

> 
> >         if (test_bit(SCMD_STATE_INFLIGHT, &cmd->state))
> >                 (*count)++;
> >  
> > diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> > index b3f14f05340a..0a9a36c349ee 100644
> > --- a/drivers/scsi/scsi_lib.c
> > +++ b/drivers/scsi/scsi_lib.c
> > @@ -1353,8 +1353,12 @@ static inline int
> > scsi_host_queue_ready(struct request_queue *q,
> >         if (scsi_host_in_recovery(shost))
> >                 return 0;
> >  
> > +       set_bit(SCMD_STATE_INFLIGHT, &cmd->state);
> > +       /* This pairs with test_bit() in
> > scsi_host_check_in_flight() */
> > +       smp_mb__after_atomic();
> > +
> >         if (atomic_read(&shost->host_blocked) > 0) {
> > -               if (scsi_host_busy(shost) > 0)
> > +               if (scsi_host_busy(shost) > 1)
> >                         goto starved;
> >  
> >                 /*
> > @@ -1379,8 +1383,6 @@ static inline int
> > scsi_host_queue_ready(struct request_queue *q,
> >                 spin_unlock_irq(shost->host_lock);
> >         }
> >  
> > -       __set_bit(SCMD_STATE_INFLIGHT, &cmd->state);
> > -
> 
> Looks this patch fine.
> 
> However, I'd suggest to confirm smartpqi's .can_queue usage first,
> which
> looks one big issue.



>