Re: [PATCH v4 5/6] IB/nes: Eliminate duplicate barriers on weakly-ordered archs

[Jason Gunthorpe]

On Tue, Mar 20, 2018 at 10:23:16AM -0500, Sinan Kaya wrote:
> On 3/20/2018 9:54 AM, Jason Gunthorpe wrote:
> > On Mon, Mar 19, 2018 at 10:47:47PM -0400, Sinan Kaya wrote:
> >> Code includes barrier() followed by writel(). writel() already has a
> >> barrier on some architectures like arm64.
> >>
> >> This ends up CPU observing two barriers back to back before executing the
> >> register write.
> >>
> >> Create a new wrapper function with relaxed write operator. Use the new
> >> wrapper when a write is following a barrier().
> >>
> >> Signed-off-by: Sinan Kaya <okaya@xxxxxxxxxxxxxx>
> >>  drivers/infiniband/hw/nes/nes.h       |  5 +++++
> >>  drivers/infiniband/hw/nes/nes_hw.c    | 21 ++++++++++++++-------
> >>  drivers/infiniband/hw/nes/nes_mgt.c   | 15 ++++++++++-----
> >>  drivers/infiniband/hw/nes/nes_nic.c   |  2 +-
> >>  drivers/infiniband/hw/nes/nes_utils.c |  3 ++-
> >>  drivers/infiniband/hw/nes/nes_verbs.c |  5 +++--
> >>  6 files changed, 35 insertions(+), 16 deletions(-)
> >>
> >> diff --git a/drivers/infiniband/hw/nes/nes.h b/drivers/infiniband/hw/nes/nes.h
> >> index 00c27291..85e007d 100644
> >> +++ b/drivers/infiniband/hw/nes/nes.h
> >> @@ -387,6 +387,11 @@ static inline void nes_write_indexed(struct nes_device *nesdev, u32 reg_index, u
> >>  	spin_unlock_irqrestore(&nesdev->indexed_regs_lock, flags);
> >>  }
> >>  
> >> +static inline void nes_write32_relaxed(void __iomem *addr, u32 val)
> >> +{
> >> +	writel_relaxed(val, addr);
> >> +}
> > 
> > This wrapper is pointless, let us not add more..
> > 
> >>  static inline void nes_write32(void __iomem *addr, u32 val)
> >>  {
> >>  	writel(val, addr);
> >> diff --git a/drivers/infiniband/hw/nes/nes_hw.c b/drivers/infiniband/hw/nes/nes_hw.c
> >> index 18a7de1..568e17d 100644
> >> +++ b/drivers/infiniband/hw/nes/nes_hw.c
> >> @@ -1257,7 +1257,8 @@ int nes_destroy_cqp(struct nes_device *nesdev)
> >>  
> >>  	barrier();
> >>  	/* Ring doorbell (5 WQEs) */
> >> -	nes_write32(nesdev->regs+NES_WQE_ALLOC, 0x05800000 | nesdev->cqp.qp_id);
> >> +	nes_write32_relaxed(nesdev->regs+NES_WQE_ALLOC,
> >> +			    0x05800000 | nesdev->cqp.qp_id);
> > 
> > barrier() is not strong enough to order writel, so this doesn't seem
> > right?
> > 
> > It is probably noteven strong enough for what this driver thinks it is
> > doing..  This driver is essentially dead and broken, probably just
> > don't change it.
> 
> Just for the sake of other changes in netdev directory and my education...
> 
> barrier() on ARM is a wmb(). It should be a compiler barrier on intel.
> 
> You are saying barrier() should have been a wmb(), right?

Yes, that is my understanding.. barrier() is supposed to be a very
weak barrier that just ensures the CPU is locally consistent with
itself. It doesn't say anything about DMA access, or SMP cases.

I don't think it is supposed to order anything related to
writel_relaxed()

> I have gone through similar exercise on netdev directory and changed
> 
> barrier()
> writel()
> 
> to 
> 
> barrier()
> writel_relaxed()
> 
> Do you see any problem with this?

Seems dangerous as a mechanical change to me, it really depends on why
the driver author put barrier() there.

In this case, I strongly suspect nes really intended to say wmb()

Jason