Re: [PATCH] drm/xe/xe_guc_ct: Prevent compiler read/write optimization breaks

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

On Tue, Dec 09, 2025 at 09:25:36PM +0000, Jonathan Cavitt wrote:
> Use READ_ONCE and WRITE_ONCE when operating on ct->state and the
> g2h_fence->done values to prevent the compiler from ignoring these
> necessary operations.
> 

Maybe a fixes tag to guard against future compiler changes breaking
older kernels?

> Suggested-by: Matthew Brost <matthew.brost@intel.com>
> Signed-off-by: Jonathan Cavitt <jonathan.cavitt@intel.com>
> ---
>  drivers/gpu/drm/xe/xe_guc_ct.c | 6 +++---
>  drivers/gpu/drm/xe/xe_guc_ct.h | 4 ++--
>  2 files changed, 5 insertions(+), 5 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_guc_ct.c b/drivers/gpu/drm/xe/xe_guc_ct.c
> index 648f0f523abb..9129c50f5370 100644
> --- a/drivers/gpu/drm/xe/xe_guc_ct.c
> +++ b/drivers/gpu/drm/xe/xe_guc_ct.c
> @@ -206,7 +206,7 @@ static void g2h_fence_cancel(struct g2h_fence *g2h_fence)
>  {
>  	g2h_fence->cancel = true;
>  	g2h_fence->fail = true;
> -	g2h_fence->done = true;
> +	WRITE_ONCE(g2h_fence->done, true);

Peer Thomas's feedback can we get pairs with comments around READ_ONCE /
WRITE_ONCE usages? Grep in Xe for READ_ONCE / WRITE_ONCE and you should
see comments that are an example of this.

Matt

>  }
>  
>  static bool g2h_fence_needs_alloc(struct g2h_fence *g2h_fence)
> @@ -527,7 +527,7 @@ 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(ct->state, state);
>  
>  	xe_gt_dbg(gt, "GuC CT communication channel %s\n",
>  		  state == XE_GUC_CT_STATE_STOPPED ? "stopped" :
> @@ -1496,7 +1496,7 @@ 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(g2h_fence->done, true);
>  	smp_mb();
>  
>  	wake_up_all(&ct->g2h_fence_wq);
> diff --git a/drivers/gpu/drm/xe/xe_guc_ct.h b/drivers/gpu/drm/xe/xe_guc_ct.h
> index 5599939f8fe1..cb1335f1d66f 100644
> --- a/drivers/gpu/drm/xe/xe_guc_ct.h
> +++ b/drivers/gpu/drm/xe/xe_guc_ct.h
> @@ -30,12 +30,12 @@ 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;
> +	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;
> +	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
>