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

[Rodrigo Vivi]

On Wed, Mar 01, 2023 at 08:36:29AM -0800, Lucas De Marchi wrote:
> 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.
> 
> I don't follow this last paragraph. Could you point it in the code?

I also didn't understand this... if we remove that we will end up in
memory access with the sleeping device...

> 
> > 
> > Signed-off-by: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> > ---
> > 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);
> 
> 
> +Matt Brost
> 
> Any reason for not using kref?

hmmm... my bad actually...

I did considered the kref, but I can't remember why I haven't used it.
I recently was asking myself the same question.

> 
> Lucas De Marchi
> 
> > 
> > -	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);

hmmm... I'm afraid this can be tricky without locks...

if we have 3 simultaneous threads calling this.
get
get
put
get

and they happened in this order but the resume didn't finished yet
on the first one, then you will:
1. end up the runtime pm twice.
2. the second will pass over thinking the gpu is already awake, but it might
be still asleep.

> > -	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
> >