[Intel-gfx] [PATCH 1/2] drm/i915/dsb: remove atomic operations

[Matt Roper]

On Mon, Nov 11, 2019 at 12:50:24PM -0800, Lucas De Marchi wrote:
> The current dsb API is not really prepared to handle multithread access.
> I was debugging an issue that ended up fixed by commit a096883dda2c
> ("drm/i915/dsb: Remove PIN_MAPPABLE from the DSB object VMA") and was
> puzzled how these atomic operations were guaranteeing atomicity.
> 
> 	if (atomic_add_return(1, &dsb->refcount) != 1)
> 		return dsb;
> 
> Thread A could still be initializing dsb struct (and even fail in the
> middle) while thread B would take a reference and use it (even
> derefencing a NULL cmd_buf).
> 
> I don't think the atomic operations here will help much if this were
> to support multithreaded scenario in future, so just remove them to
> avoid confusion.
> 
> Signed-off-by: Lucas De Marchi <lucas.demarchi at intel.com>

Agreed; the synchronization here doesn't appear to make sense.  But
also I believe everywhere this would get called is on the atomic commit
path and we already hold the CRTC lock at that point which will prevent
concurrent threads calling this.  So

Reviewed-by: Matt Roper <matthew.d.roper at intel.com>

> ---
>  drivers/gpu/drm/i915/display/intel_dsb.c | 10 +++++-----
>  drivers/gpu/drm/i915/display/intel_dsb.h |  2 +-
>  2 files changed, 6 insertions(+), 6 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/display/intel_dsb.c b/drivers/gpu/drm/i915/display/intel_dsb.c
> index d8ad5fe1efef..4feebbeb0b0c 100644
> --- a/drivers/gpu/drm/i915/display/intel_dsb.c
> +++ b/drivers/gpu/drm/i915/display/intel_dsb.c
> @@ -107,7 +107,7 @@ intel_dsb_get(struct intel_crtc *crtc)
>  	if (!HAS_DSB(i915))
>  		return dsb;
>  
> -	if (atomic_add_return(1, &dsb->refcount) != 1)
> +	if (++dsb->refcount != 1)
>  		return dsb;
>  
>  	dsb->id = DSB1;
> @@ -123,7 +123,7 @@ intel_dsb_get(struct intel_crtc *crtc)
>  	if (IS_ERR(vma)) {
>  		DRM_ERROR("Vma creation failed\n");
>  		i915_gem_object_put(obj);
> -		atomic_dec(&dsb->refcount);
> +		dsb->refcount--;
>  		goto err;
>  	}
>  
> @@ -132,7 +132,7 @@ intel_dsb_get(struct intel_crtc *crtc)
>  		DRM_ERROR("Command buffer creation failed\n");
>  		i915_vma_unpin_and_release(&vma, 0);
>  		dsb->cmd_buf = NULL;
> -		atomic_dec(&dsb->refcount);
> +		dsb->refcount--;
>  		goto err;
>  	}
>  	dsb->vma = vma;
> @@ -158,10 +158,10 @@ void intel_dsb_put(struct intel_dsb *dsb)
>  	if (!HAS_DSB(i915))
>  		return;
>  
> -	if (WARN_ON(atomic_read(&dsb->refcount) == 0))
> +	if (WARN_ON(dsb->refcount == 0))
>  		return;
>  
> -	if (atomic_dec_and_test(&dsb->refcount)) {
> +	if (--dsb->refcount == 0) {
>  		i915_vma_unpin_and_release(&dsb->vma, I915_VMA_RELEASE_MAP);
>  		dsb->cmd_buf = NULL;
>  		dsb->free_pos = 0;
> diff --git a/drivers/gpu/drm/i915/display/intel_dsb.h b/drivers/gpu/drm/i915/display/intel_dsb.h
> index 6f95c8e909e6..395ef9ce558e 100644
> --- a/drivers/gpu/drm/i915/display/intel_dsb.h
> +++ b/drivers/gpu/drm/i915/display/intel_dsb.h
> @@ -22,7 +22,7 @@ enum dsb_id {
>  };
>  
>  struct intel_dsb {
> -	atomic_t refcount;
> +	long refcount;
>  	enum dsb_id id;
>  	u32 *cmd_buf;
>  	struct i915_vma *vma;
> -- 
> 2.24.0
> 
> _______________________________________________
> Intel-gfx mailing list
> Intel-gfx at lists.freedesktop.org
> https://lists.freedesktop.org/mailman/listinfo/intel-gfx

-- 
Matt Roper
Graphics Software Engineer
VTT-OSGC Platform Enablement
Intel Corporation
(916) 356-2795