Re: [Intel-xe] [PATCH] drm/xe: Use atomic instead of mutex for xe_device_mem_access_ongoing

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

On Tue, Feb 28, 2023 at 11:17:30AM +0100, Maarten Lankhorst wrote:
> xe_guc_ct_fast_path() is called from an irq context, and cannot lock
> the mutex used by xe_device_mem_access_ongoing().
> 
> Fortunately it is easy to fix, and the atomic guarantees are good enough
> to ensure xe->mem_access.hold_rpm is set before last ref is dropped.
> 
> As far as I can tell, the runtime ref in device access should be
> killable, but don't dare to do it yet.
> 
> Signed-off-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

I think this a better fix:
https://patchwork.freedesktop.org/series/115301/

Matt

> ---
>  drivers/gpu/drm/xe/xe_device.c       | 17 ++++++++---------
>  drivers/gpu/drm/xe/xe_device.h       | 14 ++++----------
>  drivers/gpu/drm/xe/xe_device_types.h |  4 +---
>  3 files changed, 13 insertions(+), 22 deletions(-)
> 
> diff --git a/drivers/gpu/drm/xe/xe_device.c b/drivers/gpu/drm/xe/xe_device.c
> index 4eb6786b11f0..ab179b1e24c1 100644
> --- a/drivers/gpu/drm/xe/xe_device.c
> +++ b/drivers/gpu/drm/xe/xe_device.c
> @@ -237,7 +237,6 @@ struct xe_device *xe_device_create(struct pci_dev *pdev,
>  	if (err)
>  		goto err;
>  
> -	mutex_init(&xe->mem_access.lock);
>  	return xe;
>  
>  err_put:
> @@ -424,25 +423,25 @@ u32 xe_device_ccs_bytes(struct xe_device *xe, u64 size)
>  void xe_device_mem_access_get(struct xe_device *xe)
>  {
>  	bool resumed = xe_pm_runtime_resume_if_suspended(xe);
> +	int ref = atomic_inc_return(&xe->mem_access.ref);
>  
> -	mutex_lock(&xe->mem_access.lock);
> -	if (xe->mem_access.ref++ == 0)
> +	if (ref == 1)
>  		xe->mem_access.hold_rpm = xe_pm_runtime_get_if_active(xe);
> -	mutex_unlock(&xe->mem_access.lock);
>  
>  	/* The usage counter increased if device was immediately resumed */
>  	if (resumed)
>  		xe_pm_runtime_put(xe);
>  
> -	XE_WARN_ON(xe->mem_access.ref == S32_MAX);
> +	XE_WARN_ON(ref == S32_MAX);
>  }
>  
>  void xe_device_mem_access_put(struct xe_device *xe)
>  {
> -	mutex_lock(&xe->mem_access.lock);
> -	if (--xe->mem_access.ref == 0 && xe->mem_access.hold_rpm)
> +	bool hold = xe->mem_access.hold_rpm;
> +	int ref = atomic_dec_return(&xe->mem_access.ref);
> +
> +	if (!ref && hold)
>  		xe_pm_runtime_put(xe);
> -	mutex_unlock(&xe->mem_access.lock);
>  
> -	XE_WARN_ON(xe->mem_access.ref < 0);
> +	XE_WARN_ON(ref < 0);
>  }
> diff --git a/drivers/gpu/drm/xe/xe_device.h b/drivers/gpu/drm/xe/xe_device.h
> index 263620953c3b..96b4f3d7969e 100644
> --- a/drivers/gpu/drm/xe/xe_device.h
> +++ b/drivers/gpu/drm/xe/xe_device.h
> @@ -90,20 +90,14 @@ static inline struct xe_force_wake * gt_to_fw(struct xe_gt *gt)
>  void xe_device_mem_access_get(struct xe_device *xe);
>  void xe_device_mem_access_put(struct xe_device *xe);
>  
> -static inline void xe_device_assert_mem_access(struct xe_device *xe)
> +static inline bool xe_device_mem_access_ongoing(struct xe_device *xe)
>  {
> -	XE_WARN_ON(!xe->mem_access.ref);
> +	return atomic_read(&xe->mem_access.ref);
>  }
>  
> -static inline bool xe_device_mem_access_ongoing(struct xe_device *xe)
> +static inline void xe_device_assert_mem_access(struct xe_device *xe)
>  {
> -	bool ret;
> -
> -	mutex_lock(&xe->mem_access.lock);
> -	ret = xe->mem_access.ref;
> -	mutex_unlock(&xe->mem_access.lock);
> -
> -	return ret;
> +	XE_WARN_ON(!xe_device_mem_access_ongoing(xe));
>  }
>  
>  static inline bool xe_device_in_fault_mode(struct xe_device *xe)
> diff --git a/drivers/gpu/drm/xe/xe_device_types.h b/drivers/gpu/drm/xe/xe_device_types.h
> index 9743987fc883..0b8c4ee0ad48 100644
> --- a/drivers/gpu/drm/xe/xe_device_types.h
> +++ b/drivers/gpu/drm/xe/xe_device_types.h
> @@ -230,10 +230,8 @@ struct xe_device {
>  	 * triggering additional actions when they occur.
>  	 */
>  	struct {
> -		/** @lock: protect the ref count */
> -		struct mutex lock;
>  		/** @ref: ref count of memory accesses */
> -		s32 ref;
> +		atomic_t ref;
>  		/** @hold_rpm: need to put rpm ref back at the end */
>  		bool hold_rpm;
>  	} mem_access;
> -- 
> 2.34.1
>