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@xxxxxxxxx>
Reviewed-by: Matt Roper <matthew.d.roper@xxxxxxxxx>
Link: https://patchwork.freedesktop.org/patch/msgid/20191111205024.22853-2-lucas.demarchi@xxxxxxxxx
---
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
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