Re: [PATCH v3 2/2] drm/xe/guc: READ/WRITE_ONCE ct->state

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

On Mon, Dec 22, 2025 at 08:20:00PM +0000, Jonathan Cavitt wrote:
> Use READ_ONCE and WRITE_ONCE when operating on ct->state
> to prevent the compiler form ignoring important modifications
> to its value.
> 
> 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>
> Cc: Michal Wajdeczko <michal.wajdeczko@intel.com>
> Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_guc_ct.c | 7 ++++++-
>  drivers/gpu/drm/xe/xe_guc_ct.h | 6 ++++--
>  2 files changed, 10 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_guc_ct.c b/drivers/gpu/drm/xe/xe_guc_ct.c
> index f2148a8bf208..dfbf76037b04 100644
> --- a/drivers/gpu/drm/xe/xe_guc_ct.c
> +++ b/drivers/gpu/drm/xe/xe_guc_ct.c
> @@ -529,7 +529,12 @@ static void guc_ct_change_state(struct xe_guc_ct *ct,
>  	if (ct->g2h_outstanding)
>  		xe_pm_runtime_put(ct_to_xe(ct));
>  	ct->g2h_outstanding = 0;
> -	ct->state = state;
> +
> +	/*
> +	 * WRITE_ONCE pairs with READ_ONCEs in xe_guc_ct_initialized and
> +	 * xe_guc_ct_enabled.
> +	 */
> +	WRITE_ONCE(ct->state, state);
>  
>  	xe_gt_dbg(gt, "GuC CT communication channel %s\n",
>  		  state == XE_GUC_CT_STATE_STOPPED ? "stopped" :
> diff --git a/drivers/gpu/drm/xe/xe_guc_ct.h b/drivers/gpu/drm/xe/xe_guc_ct.h
> index 5599939f8fe1..767365a33dee 100644
> --- a/drivers/gpu/drm/xe/xe_guc_ct.h
> +++ b/drivers/gpu/drm/xe/xe_guc_ct.h
> @@ -30,12 +30,14 @@ void xe_guc_ct_print(struct xe_guc_ct *ct, struct drm_printer *p, bool want_ctb)
>  
>  static inline bool xe_guc_ct_initialized(struct xe_guc_ct *ct)
>  {
> -	return ct->state != XE_GUC_CT_STATE_NOT_INITIALIZED;
> +	/* READ_ONCE pairs with WRITE_ONCE in guc_ct_change_state. */
> +	return READ_ONCE(ct->state) != XE_GUC_CT_STATE_NOT_INITIALIZED;
>  }
>  
>  static inline bool xe_guc_ct_enabled(struct xe_guc_ct *ct)
>  {
> -	return ct->state == XE_GUC_CT_STATE_ENABLED;
> +	/* READ_ONCE pairs with WRITE_ONCE in guc_ct_change_state. */
> +	return READ_ONCE(ct->state) == XE_GUC_CT_STATE_ENABLED;
>  }
>  
>  static inline void xe_guc_ct_irq_handler(struct xe_guc_ct *ct)
> -- 
> 2.43.0
>