[v7,05/28] dma-heap: Add proper kref handling on dma-buf heaps

Commit Message

Yunfei Dong July 20, 2024, 7:15 a.m. UTC

From: John Stultz <[e-mail]>

Add proper reference counting on the dma_heap structure. While
existing heaps are built-in, we may eventually have heaps loaded
from modules, and we'll need to be able to properly handle the
references to the heaps

Signed-off-by: John Stultz <[e-mail]>
Signed-off-by: T.J. Mercier <[e-mail]>
Signed-off-by: Yong Wu <[e-mail]>
[Yong: Just add comment for "minor" and "refcount"]
Signed-off-by: Yunfei Dong <[e-mail]>
[Yunfei: Change reviewer's comments]
---
 drivers/dma-buf/dma-heap.c | 29 +++++++++++++++++++++++++++++
 include/linux/dma-heap.h   |  2 ++
 2 files changed, 31 insertions(+)

Comments

Markus Elfring July 20, 2024, 3:13 p.m. UTC | #1

…
> +++ b/drivers/dma-buf/dma-heap.c
…
> +static void dma_heap_release(struct kref *ref)
> +{
…
> +	mutex_lock(&heap_list_lock);
> +	list_del(&heap->list);
> +	mutex_unlock(&heap_list_lock);
…

Under which circumstances would you become interested to apply a statement
like “guard(mutex)(&heap_list_lock);”?
https://elixir.bootlin.com/linux/v6.10/source/include/linux/mutex.h#L196

Regards,
Markus

John Stultz July 22, 2024, 6:06 p.m. UTC | #2

On Sat, Jul 20, 2024 at 8:13 AM Markus Elfring <[e-mail]> wrote:
>
> …
> > +++ b/drivers/dma-buf/dma-heap.c
> …
> > +static void dma_heap_release(struct kref *ref)
> > +{
> …
> > +     mutex_lock(&heap_list_lock);
> > +     list_del(&heap->list);
> > +     mutex_unlock(&heap_list_lock);
> …
>
> Under which circumstances would you become interested to apply a statement
> like “guard(mutex)(&heap_list_lock);”?
> https://elixir.bootlin.com/linux/v6.10/source/include/linux/mutex.h#L196

This strikes me as a strange place to apply it, as it seems like it
would grow the lock hold time to the entire scope of the function
unless one created a subscope for just the list_del, at which point
you're not saving much or really improving readability.  I definitely
think guard usage is very interesting in places where locks are
released in multiple exit paths, etc. but this is a very trivial and
straightforward lock/unlock usage, so I fret I don't quite understand
the suggestion.

thanks
-john

Markus Elfring July 22, 2024, 6:38 p.m. UTC | #3

>> …
>>> +++ b/drivers/dma-buf/dma-heap.c
>> …
>>> +static void dma_heap_release(struct kref *ref)
>>> +{
>> …
>>> +     mutex_lock(&heap_list_lock);
>>> +     list_del(&heap->list);
>>> +     mutex_unlock(&heap_list_lock);
>> …
>>
>> Under which circumstances would you become interested to apply a statement
>> like “guard(mutex)(&heap_list_lock);”?
>> https://elixir.bootlin.com/linux/v6.10/source/include/linux/mutex.h#L196
>
> This strikes me as a strange place to apply it, as it seems like it
> would grow the lock hold time to the entire scope of the function
> unless one created a subscope for just the list_del, at which point
> you're not saving much or really improving readability.  I definitely
> think guard usage is very interesting in places where locks are
> released in multiple exit paths, etc. but this is a very trivial and
> straightforward lock/unlock usage, so I fret I don't quite understand
> the suggestion.

I propose to take further design possibilities better into account for
applications of scope-based resource management.
Additional compound statements may be constructed on demand by adding
extra curly brackets.
You might occasionally find scoped guards more appealing.
https://elixir.bootlin.com/linux/v6.10/source/include/linux/cleanup.h#L137

Regards,
Markus

Patch

diff --git a/drivers/dma-buf/dma-heap.c b/drivers/dma-buf/dma-heap.c
index 22f6c193db0d..97025ee8500f 100644
--- a/drivers/dma-buf/dma-heap.c
+++ b/drivers/dma-buf/dma-heap.c
@@ -11,6 +11,7 @@ 
 #include <linux/dma-buf.h>
 #include <linux/dma-heap.h>
 #include <linux/err.h>
+#include <linux/kref.h>
 #include <linux/list.h>
 #include <linux/nospec.h>
 #include <linux/syscalls.h>
@@ -30,6 +31,7 @@ 
  * @heap_devt:		heap device node
  * @list:		list head connecting to list of heaps
  * @heap_cdev:		heap char device
+ * @refcount:		reference counter for this heap device
  *
  * Represents a heap of memory from which buffers can be made.
  */
@@ -40,6 +42,7 @@  struct dma_heap {
 	dev_t heap_devt;
 	struct list_head list;
 	struct cdev heap_cdev;
+	struct kref refcount;
 };
 
 static LIST_HEAD(heap_list);
@@ -240,6 +243,7 @@  struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
 	if (!heap)
 		return ERR_PTR(-ENOMEM);
 
+	kref_init(&heap->refcount);
 	heap->name = exp_info->name;
 	heap->ops = exp_info->ops;
 	heap->priv = exp_info->priv;
@@ -304,6 +308,31 @@  struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
 	return err_ret;
 }
 
+static void dma_heap_release(struct kref *ref)
+{
+	struct dma_heap *heap = container_of(ref, struct dma_heap, refcount);
+	unsigned int minor = MINOR(heap->heap_devt);
+
+	mutex_lock(&heap_list_lock);
+	list_del(&heap->list);
+	mutex_unlock(&heap_list_lock);
+
+	device_destroy(dma_heap_class, heap->heap_devt);
+	cdev_del(&heap->heap_cdev);
+	xa_erase(&dma_heap_minors, minor);
+
+	kfree(heap);
+}
+
+/**
+ * dma_heap_put - drops a reference to a dmabuf heap, potentially freeing it
+ * @heap: DMA-Heap whose reference count to decrement
+ */
+void dma_heap_put(struct dma_heap *heap)
+{
+	kref_put(&heap->refcount, dma_heap_release);
+}
+
 static char *dma_heap_devnode(const struct device *dev, umode_t *mode)
 {
 	return kasprintf(GFP_KERNEL, "dma_heap/%s", dev_name(dev));
diff --git a/include/linux/dma-heap.h b/include/linux/dma-heap.h
index fbe86ec889a8..d57593f8a1bc 100644
--- a/include/linux/dma-heap.h
+++ b/include/linux/dma-heap.h
@@ -46,4 +46,6 @@  const char *dma_heap_get_name(struct dma_heap *heap);
 
 struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info);
 
+void dma_heap_put(struct dma_heap *heap);
+
 #endif /* _DMA_HEAPS_H */