Re: [PATCH for-next 07/14] IB/mlx5: Make sure doorbell record is visible before doorbell

[Yann Droneaud <ydroneaud-RlY5vtjFyJ3QT0dZR+AlfA@public.gmane.org>]

Le mardi 14 janvier 2014 à 17:45 +0200, Eli Cohen a écrit :
> Put a wmb() to make sure the doorbell record is visible to the HCA before we
> hit doorbell.
> 
> Signed-off-by: Eli Cohen <eli-VPRAkNaXOzVWk0Htik3J/w@public.gmane.org>
> ---
>  drivers/infiniband/hw/mlx5/qp.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/drivers/infiniband/hw/mlx5/qp.c b/drivers/infiniband/hw/mlx5/qp.c
> index a056c24..87b7fb1 100644
> --- a/drivers/infiniband/hw/mlx5/qp.c
> +++ b/drivers/infiniband/hw/mlx5/qp.c
> @@ -2251,6 +2251,10 @@ out:
>  
>  		qp->db.db[MLX5_SND_DBR] = cpu_to_be32(qp->sq.cur_post);
>  
> +		/* Make sure doorbell record is visible to the HCA before
> +		 * we hit doorbell */
> +		wmb();
> +

In linux-next, as of next-20140114, I'm seeing the write memory barrier
before "qp->db.db[MLX5_SND_DBR] = cpu_to_be32(qp->sq.cur_post);"
It was introduced by commit e126ba97dba9e:

out:
        if (likely(nreq)) {
                qp->sq.head += nreq;

                /* Make sure that descriptors are written before
                 * updating doorbell record and ringing the doorbell
                 */
                wmb();

                qp->db.db[MLX5_SND_DBR] = cpu_to_be32(qp->sq.cur_post);

                if (bf->need_lock)
                        spin_lock(&bf->lock);

So this add a second wmb().
Is it really necessary ?

>  		if (bf->need_lock)
>  			spin_lock(&bf->lock);
>  

Regards.

-- 
Yann Droneaud
OPTEYA


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