[Intel-gfx] [PATCH v2] drm/i915/dsb: remove atomic operations

[Lucas De Marchi]

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.

v2: Use refcount++ != 0 instead of ++refcount != 1 (from Ville)

Signed-off-by: Lucas De Marchi <lucas.demarchi at intel.com>
Reviewed-by: Matt Roper <matthew.d.roper at intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191111205024.22853-2-lucas.demarchi@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..50c4d98c0020 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++ != 0)
 		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