Re: [Intel-xe] [PATCH v4 2/7] drm/xe/ct: hold fast_lock when reserving space for g2h

[Matthew Brost <matthew.brost@intel.com>]

On Wed, Jul 05, 2023 at 05:06:05PM +0100, Matthew Auld wrote:
> Reserving and checking for space on the g2h side relies on the
> fast_lock, and not the CT lock since we need to release space from the
> fast CT path. Make sure we hold it when checking for space and reserving
> it. The main concern is calling __g2h_release_space() as we are reserving
> something and since the info.space and info.g2h_outstanding operations
> are not atomic we can get some nonsense values back.
> 
> Signed-off-by: Matthew Auld <matthew.auld@intel.com>
> Cc: Matthew Brost <matthew.brost@intel.com>

Reviewed-by: Matthew Brost <matthew.brost@intel.com>

> Cc: José Roberto de Souza <jose.souza@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_guc_ct.c | 22 +++++++++++++++-------
>  1 file changed, 15 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_guc_ct.c b/drivers/gpu/drm/xe/xe_guc_ct.c
> index b7aecc480098..f8c1a2ca89f7 100644
> --- a/drivers/gpu/drm/xe/xe_guc_ct.c
> +++ b/drivers/gpu/drm/xe/xe_guc_ct.c
> @@ -346,7 +346,10 @@ static bool h2g_has_room(struct xe_guc_ct *ct, u32 cmd_len)
>  
>  static bool g2h_has_room(struct xe_guc_ct *ct, u32 g2h_len)
>  {
> -	lockdep_assert_held(&ct->lock);
> +	if (!g2h_len)
> +		return true;
> +
> +	lockdep_assert_held(&ct->fast_lock);
>  
>  	return ct->ctbs.g2h.info.space > g2h_len;
>  }
> @@ -367,15 +370,15 @@ static void h2g_reserve_space(struct xe_guc_ct *ct, u32 cmd_len)
>  	ct->ctbs.h2g.info.space -= cmd_len;
>  }
>  
> -static void g2h_reserve_space(struct xe_guc_ct *ct, u32 g2h_len, u32 num_g2h)
> +static void __g2h_reserve_space(struct xe_guc_ct *ct, u32 g2h_len, u32 num_g2h)
>  {
>  	XE_BUG_ON(g2h_len > ct->ctbs.g2h.info.space);
>  
>  	if (g2h_len) {
> -		spin_lock_irq(&ct->fast_lock);
> +		lockdep_assert_held(&ct->fast_lock);
> +
>  		ct->ctbs.g2h.info.space -= g2h_len;
>  		ct->g2h_outstanding += num_g2h;
> -		spin_unlock_irq(&ct->fast_lock);
>  	}
>  }
>  
> @@ -499,21 +502,26 @@ static int __guc_ct_send_locked(struct xe_guc_ct *ct, const u32 *action,
>  		}
>  	}
>  
> +	if (g2h_len)
> +		spin_lock_irq(&ct->fast_lock);
>  retry:
>  	ret = has_room(ct, len + GUC_CTB_HDR_LEN, g2h_len);
>  	if (unlikely(ret))
> -		goto out;
> +		goto out_unlock;
>  
>  	ret = h2g_write(ct, action, len, g2h_fence ? g2h_fence->seqno : 0,
>  			!!g2h_fence);
>  	if (unlikely(ret)) {
>  		if (ret == -EAGAIN)
>  			goto retry;
> -		goto out;
> +		goto out_unlock;
>  	}
>  
> -	g2h_reserve_space(ct, g2h_len, num_g2h);
> +	__g2h_reserve_space(ct, g2h_len, num_g2h);
>  	xe_guc_notify(ct_to_guc(ct));
> +out_unlock:
> +	if (g2h_len)
> +		spin_unlock_irq(&ct->fast_lock);
>  out:
>  	return ret;
>  }
> -- 
> 2.41.0
>