Re: [PATCH v3 1/2] drm/xe/guc: READ/WRITE_ONCE g2h_fence->done

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

On Mon, Dec 22, 2025 at 08:19:59PM +0000, Jonathan Cavitt wrote:
> Use READ_ONCE and WRITE_ONCE when operating on g2h_fence->done
> to prevent the compiler from ignoring important modifications
> to its value.
> 
> Fixes: dd08ebf6c352 ("drm/xe: Introduce a new DRM driver for Intel GPUs")
> Suggested-by: Matthew Brost <matthew.brost@intel.com>

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

> Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
> Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_guc_ct.c | 14 ++++++++++----
>  1 file changed, 10 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_guc_ct.c b/drivers/gpu/drm/xe/xe_guc_ct.c
> index c3df9b3f1b4d..f2148a8bf208 100644
> --- a/drivers/gpu/drm/xe/xe_guc_ct.c
> +++ b/drivers/gpu/drm/xe/xe_guc_ct.c
> @@ -206,7 +206,9 @@ static void g2h_fence_cancel(struct g2h_fence *g2h_fence)
>  {
>  	g2h_fence->cancel = true;
>  	g2h_fence->fail = true;
> -	g2h_fence->done = true;
> +
> +	/* WRITE_ONCE pairs with READ_ONCEs in guc_ct_send_recv. */
> +	WRITE_ONCE(g2h_fence->done, true);
>  }
>  
>  static bool g2h_fence_needs_alloc(struct g2h_fence *g2h_fence)
> @@ -1294,10 +1296,13 @@ static int guc_ct_send_recv(struct xe_guc_ct *ct, const u32 *action, u32 len,
>  		return ret;
>  	}
>  
> -	ret = wait_event_timeout(ct->g2h_fence_wq, g2h_fence.done, HZ);
> +	/* READ_ONCEs pairs with WRITE_ONCEs in parse_g2h_response
> +	 * and g2h_fence_cancel.
> +	 */
> +	ret = wait_event_timeout(ct->g2h_fence_wq, READ_ONCE(g2h_fence.done), HZ);
>  	if (!ret) {
>  		LNL_FLUSH_WORK(&ct->g2h_worker);
> -		if (g2h_fence.done) {
> +		if (READ_ONCE(g2h_fence.done)) {
>  			xe_gt_warn(gt, "G2H fence %u, action %04x, done\n",
>  				   g2h_fence.seqno, action[0]);
>  			ret = 1;
> @@ -1498,7 +1503,8 @@ static int parse_g2h_response(struct xe_guc_ct *ct, u32 *msg, u32 len)
>  
>  	g2h_release_space(ct, GUC_CTB_HXG_MSG_MAX_LEN);
>  
> -	g2h_fence->done = true;
> +	/* WRITE_ONCE pairs with READ_ONCEs in guc_ct_send_recv. */
> +	WRITE_ONCE(g2h_fence->done, true);
>  	smp_mb();
>  
>  	wake_up_all(&ct->g2h_fence_wq);
> -- 
> 2.43.0
>