Re: [PATCH 2/3] scsi: make sure that request queue queiesce and unquiesce balanced

[James Bottomley <James.Bottomley@HansenPartnership.com>]

On Tue, 2021-11-02 at 06:59 -0600, Jens Axboe wrote:
> On 11/1/21 7:43 PM, James Bottomley wrote:
> > On Thu, 2021-10-21 at 22:59 +0800, Ming Lei wrote:
> > > For fixing queue quiesce race between driver and block
> > > layer(elevator switch, update nr_requests, ...), we need to
> > > support concurrent quiesce and unquiesce, which requires the two
> > > call balanced.
> > > 
> > > It isn't easy to audit that in all scsi drivers, especially the
> > > two may be called from different contexts, so do it in scsi core
> > > with one per-device bit flag & global spinlock, basically zero
> > > cost since request queue quiesce is seldom triggered.
> > > 
> > > Reported-by: Yi Zhang <yi.zhang@redhat.com>
> > > Fixes: e70feb8b3e68 ("blk-mq: support concurrent queue
> > > quiesce/unquiesce")
> > > Signed-off-by: Ming Lei <ming.lei@redhat.com>
> > > ---
> > >  drivers/scsi/scsi_lib.c    | 45 ++++++++++++++++++++++++++++++
> > > ----
> > > ----
> > >  include/scsi/scsi_device.h |  1 +
> > >  2 files changed, 37 insertions(+), 9 deletions(-)
> > > 
> > > diff --git a/drivers/scsi/scsi_lib.c b/drivers/scsi/scsi_lib.c
> > > index 51fcd46be265..414f4daf8005 100644
> > > --- a/drivers/scsi/scsi_lib.c
> > > +++ b/drivers/scsi/scsi_lib.c
> > > @@ -2638,6 +2638,40 @@ static int
> > > __scsi_internal_device_block_nowait(struct scsi_device *sdev)
> > >  	return 0;
> > >  }
> > >  
> > > +static DEFINE_SPINLOCK(sdev_queue_stop_lock);
> > > +
> > > +void scsi_start_queue(struct scsi_device *sdev)
> > > +{
> > > +	bool need_start;
> > > +	unsigned long flags;
> > > +
> > > +	spin_lock_irqsave(&sdev_queue_stop_lock, flags);
> > > +	need_start = sdev->queue_stopped;
> > > +	sdev->queue_stopped = 0;
> > > +	spin_unlock_irqrestore(&sdev_queue_stop_lock, flags);
> > > +
> > > +	if (need_start)
> > > +		blk_mq_unquiesce_queue(sdev->request_queue);
> > 
> > Well, this is a classic atomic pattern:
> > 
> > if (cmpxchg(&sdev->queue_stopped, 1, 0))
> > 	blk_mq_unquiesce_queue(sdev->request_queue);
> > 
> > The reason to do it with atomics rather than spinlocks is
> > 
> >    1. no need to disable interrupts: atomics are locked
> >    2. faster because a spinlock takes an exclusive line every time
> > but the
> >       read to check the value can be in shared mode in cmpxchg
> >    3. it's just shorter and better code.
> > 
> > The only minor downside is queue_stopped now needs to be a u32.
> 
> Are you fine with the change as-is, or do you want it redone? I
> can drop the SCSI parts and just queue up the dm fix. Personally
> I think it'd be better to get it fixed upfront.

Well, given the path isn't hot, I don't really care.  However, what I
don't want is to have to continually bat back patches from the make
work code churners trying to update this code for being the wrong
pattern.  I think at the very least it needs a comment saying why we
chose a suboptimal pattern to try to forestall this.

James