[PATCH 17/20] drm/prime: proper locking+refcounting for obj->dma_buf link
Daniel Vetter
daniel.vetter at ffwll.ch
Wed Aug 14 15:02:46 PDT 2013
The export dma-buf cache is semantically similar to an flink name. So
semantically it makes sense to treat it the same and remove the name
(i.e. the dma_buf pointer) and its references when the last gem handle
disappears.
Again we need to be careful, but double so: Not just could someone
race and export with a gem close ioctl (so we need to recheck
obj->handle_count again when assigning the new name), but multiple
exports can also race against each another. This is prevented by
holding the dev->object_name_lock across the entire section which
touches obj->dma_buf.
With the new scheme we also need to reinstate the obj->dma_buf link at
import time (in case the only reference userspace has held in-between
was through the dma-buf fd and not through any native gem handle). For
simplicity we don't check whether it's a native object but
unconditionally set up that link - with the new scheme of removing the
obj->dma_buf reference when the last handle disappears we can do that.
To make it clear that this is not just for exported buffers anymore
als rename it from export_dma_buf to dma_buf.
To make sure that now one can race a fd_to_handle or handle_to_fd with
gem_close we use the same tricks as in flink of extending the
dev->object_name_locking critical section. With this change we finally
have a guaranteed 1:1 relationship (at least for native objects)
between gem objects and dma-bufs, even accounting for races (which can
happen since the dma-buf itself holds a reference while in-flight).
This prevent igt/prime_self_import/export-vs-gem_close-race from
Oopsing the kernel. There is still a leak though since the per-file
priv dma-buf/handle cache handling is racy. That will be fixed in a
later patch.
v2: Remove the bogus dma_buf_put from the export_and_register_object
failure path if we've raced with the handle count dropping to 0.
Signed-off-by: Daniel Vetter <daniel.vetter at ffwll.ch>
---
drivers/gpu/drm/drm_fops.c | 1 +
drivers/gpu/drm/drm_gem.c | 24 ++++++++++++++--
drivers/gpu/drm/drm_prime.c | 70 +++++++++++++++++++++++++++++++++++----------
include/drm/drmP.h | 12 ++++++--
4 files changed, 87 insertions(+), 20 deletions(-)
diff --git a/drivers/gpu/drm/drm_fops.c b/drivers/gpu/drm/drm_fops.c
index 72acae9..faf93ff3 100644
--- a/drivers/gpu/drm/drm_fops.c
+++ b/drivers/gpu/drm/drm_fops.c
@@ -545,6 +545,7 @@ int drm_release(struct inode *inode, struct file *filp)
if (dev->driver->postclose)
dev->driver->postclose(dev, file_priv);
+
if (drm_core_check_feature(dev, DRIVER_PRIME))
drm_prime_destroy_file_private(&file_priv->prime);
diff --git a/drivers/gpu/drm/drm_gem.c b/drivers/gpu/drm/drm_gem.c
index 0e9a9c7..6b2b54e 100644
--- a/drivers/gpu/drm/drm_gem.c
+++ b/drivers/gpu/drm/drm_gem.c
@@ -195,9 +195,14 @@ drm_gem_remove_prime_handles(struct drm_gem_object *obj, struct drm_file *filp)
drm_prime_remove_buf_handle(&filp->prime,
obj->import_attach->dmabuf);
}
- if (obj->export_dma_buf) {
+
+ /*
+ * Note: obj->dma_buf can't disappear as long as we still hold a
+ * handle reference in obj->handle_count.
+ */
+ if (obj->dma_buf) {
drm_prime_remove_buf_handle(&filp->prime,
- obj->export_dma_buf);
+ obj->dma_buf);
}
}
@@ -231,6 +236,15 @@ static void drm_gem_object_handle_free(struct drm_gem_object *obj)
}
}
+static void drm_gem_object_exported_dma_buf_free(struct drm_gem_object *obj)
+{
+ /* Unbreak the reference cycle if we have an exported dma_buf. */
+ if (obj->dma_buf) {
+ dma_buf_put(obj->dma_buf);
+ obj->dma_buf = NULL;
+ }
+}
+
static void
drm_gem_object_handle_unreference_unlocked(struct drm_gem_object *obj)
{
@@ -244,8 +258,10 @@ drm_gem_object_handle_unreference_unlocked(struct drm_gem_object *obj)
*/
mutex_lock(&obj->dev->object_name_lock);
- if (--obj->handle_count == 0)
+ if (--obj->handle_count == 0) {
drm_gem_object_handle_free(obj);
+ drm_gem_object_exported_dma_buf_free(obj);
+ }
mutex_unlock(&obj->dev->object_name_lock);
drm_gem_object_unreference_unlocked(obj);
@@ -589,6 +605,8 @@ drm_gem_release(struct drm_device *dev, struct drm_file *file_private)
void
drm_gem_object_release(struct drm_gem_object *obj)
{
+ WARN_ON(obj->dma_buf);
+
if (obj->filp)
fput(obj->filp);
}
diff --git a/drivers/gpu/drm/drm_prime.c b/drivers/gpu/drm/drm_prime.c
index 3d57601..5e543e9 100644
--- a/drivers/gpu/drm/drm_prime.c
+++ b/drivers/gpu/drm/drm_prime.c
@@ -193,11 +193,8 @@ void drm_gem_dmabuf_release(struct dma_buf *dma_buf)
{
struct drm_gem_object *obj = dma_buf->priv;
- if (obj->export_dma_buf == dma_buf) {
- /* drop the reference on the export fd holds */
- obj->export_dma_buf = NULL;
- drm_gem_object_unreference_unlocked(obj);
- }
+ /* drop the reference on the export fd holds */
+ drm_gem_object_unreference_unlocked(obj);
}
EXPORT_SYMBOL(drm_gem_dmabuf_release);
@@ -298,6 +295,37 @@ struct dma_buf *drm_gem_prime_export(struct drm_device *dev,
}
EXPORT_SYMBOL(drm_gem_prime_export);
+static struct dma_buf *export_and_register_object(struct drm_device *dev,
+ struct drm_gem_object *obj,
+ uint32_t flags)
+{
+ struct dma_buf *dmabuf;
+
+ /* prevent races with concurrent gem_close. */
+ if (obj->handle_count == 0) {
+ dmabuf = ERR_PTR(-ENOENT);
+ return dmabuf;
+ }
+
+ dmabuf = dev->driver->gem_prime_export(dev, obj, flags);
+ if (IS_ERR(dmabuf)) {
+ /* normally the created dma-buf takes ownership of the ref,
+ * but if that fails then drop the ref
+ */
+ return dmabuf;
+ }
+
+ /*
+ * Note that callers do not need to clean up the export cache
+ * since the check for obj->handle_count guarantees that someone
+ * will clean it up.
+ */
+ obj->dma_buf = dmabuf;
+ get_dma_buf(obj->dma_buf);
+
+ return dmabuf;
+}
+
int drm_gem_prime_handle_to_fd(struct drm_device *dev,
struct drm_file *file_priv, uint32_t handle, uint32_t flags,
int *prime_fd)
@@ -313,15 +341,20 @@ int drm_gem_prime_handle_to_fd(struct drm_device *dev,
/* re-export the original imported object */
if (obj->import_attach) {
dmabuf = obj->import_attach->dmabuf;
+ get_dma_buf(dmabuf);
goto out_have_obj;
}
- if (obj->export_dma_buf) {
- dmabuf = obj->export_dma_buf;
+ mutex_lock(&dev->object_name_lock);
+ if (obj->dma_buf) {
+ get_dma_buf(obj->dma_buf);
+ dmabuf = obj->dma_buf;
+ mutex_unlock(&dev->object_name_lock);
goto out_have_obj;
}
- dmabuf = dev->driver->gem_prime_export(dev, obj, flags);
+ dmabuf = export_and_register_object(dev, obj, flags);
+ mutex_unlock(&dev->object_name_lock);
if (IS_ERR(dmabuf)) {
/* normally the created dma-buf takes ownership of the ref,
* but if that fails then drop the ref
@@ -329,14 +362,13 @@ int drm_gem_prime_handle_to_fd(struct drm_device *dev,
ret = PTR_ERR(dmabuf);
goto out;
}
- obj->export_dma_buf = dmabuf;
mutex_lock(&file_priv->prime.lock);
/* if we've exported this buffer the cheat and add it to the import list
* so we get the correct handle back
*/
ret = drm_prime_add_buf_handle(&file_priv->prime,
- obj->export_dma_buf, handle);
+ dmabuf, handle);
if (ret)
goto fail_put_dmabuf;
@@ -349,7 +381,6 @@ int drm_gem_prime_handle_to_fd(struct drm_device *dev,
return 0;
out_have_obj:
- get_dma_buf(dmabuf);
ret = dma_buf_fd(dmabuf, flags);
if (ret < 0) {
dma_buf_put(dmabuf);
@@ -365,8 +396,6 @@ fail_rm_handle:
dmabuf);
mutex_unlock(&file_priv->prime.lock);
fail_put_dmabuf:
- /* clear NOT to be checked when releasing dma_buf */
- obj->export_dma_buf = NULL;
dma_buf_put(dmabuf);
out:
drm_gem_object_unreference_unlocked(obj);
@@ -448,13 +477,22 @@ int drm_gem_prime_fd_to_handle(struct drm_device *dev,
goto out_put;
/* never seen this one, need to import */
+ mutex_lock(&dev->object_name_lock);
obj = dev->driver->gem_prime_import(dev, dma_buf);
if (IS_ERR(obj)) {
ret = PTR_ERR(obj);
- goto out_put;
+ goto out_unlock;
+ }
+
+ if (obj->dma_buf) {
+ WARN_ON(obj->dma_buf != dma_buf);
+ } else {
+ obj->dma_buf = dma_buf;
+ get_dma_buf(dma_buf);
}
- ret = drm_gem_handle_create(file_priv, obj, handle);
+ /* drm_gem_handle_create_tail unlocks dev->object_name_lock. */
+ ret = drm_gem_handle_create_tail(file_priv, obj, handle);
drm_gem_object_unreference_unlocked(obj);
if (ret)
goto out_put;
@@ -475,6 +513,8 @@ fail:
* to detach.. which seems ok..
*/
drm_gem_handle_delete(file_priv, *handle);
+out_unlock:
+ mutex_lock(&dev->object_name_lock);
out_put:
dma_buf_put(dma_buf);
mutex_unlock(&file_priv->prime.lock);
diff --git a/include/drm/drmP.h b/include/drm/drmP.h
index 3b346bc..1db42c0 100644
--- a/include/drm/drmP.h
+++ b/include/drm/drmP.h
@@ -683,8 +683,16 @@ struct drm_gem_object {
void *driver_private;
- /* dma buf exported from this GEM object */
- struct dma_buf *export_dma_buf;
+ /**
+ * dma_buf - dma buf associated with this GEM object
+ *
+ * Pointer to the dma-buf associated with this gem object (either
+ * through importing or exporting). We break the resulting reference
+ * loop when the last gem handle for this object is released.
+ *
+ * Protected by obj->object_name_lock
+ */
+ struct dma_buf *dma_buf;
/**
* import_attach - dma buf attachment backing this object
--
1.8.3.2
More information about the dri-devel
mailing list