Re: [PATCH 2/9] dma-heap: Add proper kref handling on dma-buf heaps

[T.J. Mercier]

On Mon, Sep 11, 2023 at 2:49 AM Christian König
<christian.koenig@xxxxxxx> wrote:
>
> Am 11.09.23 um 04:30 schrieb Yong Wu:
> > From: John Stultz <jstultz@xxxxxxxxxx>
> >
> > Add proper refcounting 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
> >
> > Also moves minor tracking into the heap structure so
> > we can properly free things.
>
> This is completely unnecessary, see below.
>
> >
> > Signed-off-by: John Stultz <jstultz@xxxxxxxxxx>
> > Signed-off-by: T.J. Mercier <tjmercier@xxxxxxxxxx>
> > Signed-off-by: Yong Wu <yong.wu@xxxxxxxxxxxx>
> > [Yong: Just add comment for "minor" and "refcount"]
> > ---
> >   drivers/dma-buf/dma-heap.c | 38 ++++++++++++++++++++++++++++++++++----
> >   include/linux/dma-heap.h   |  6 ++++++
> >   2 files changed, 40 insertions(+), 4 deletions(-)
> >
> > diff --git a/drivers/dma-buf/dma-heap.c b/drivers/dma-buf/dma-heap.c
> > index 51030f6c9d6e..dcc0e38c61fa 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 +31,8 @@
> >    * @heap_devt:              heap device node
> >    * @list:           list head connecting to list of heaps
> >    * @heap_cdev:              heap char device
> > + * @minor:           heap device node minor number
> > + * @refcount:                reference counter for this heap device
> >    *
> >    * Represents a heap of memory from which buffers can be made.
> >    */
> > @@ -40,6 +43,8 @@ struct dma_heap {
> >       dev_t heap_devt;
> >       struct list_head list;
> >       struct cdev heap_cdev;
> > +     int minor;
> > +     struct kref refcount;
> >   };
> >
> >   static LIST_HEAD(heap_list);
> > @@ -205,7 +210,6 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
> >   {
> >       struct dma_heap *heap, *h, *err_ret;
> >       struct device *dev_ret;
> > -     unsigned int minor;
> >       int ret;
> >
> >       if (!exp_info->name || !strcmp(exp_info->name, "")) {
> > @@ -222,12 +226,13 @@ 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;
> >
> >       /* Find unused minor number */
> > -     ret = xa_alloc(&dma_heap_minors, &minor, heap,
> > +     ret = xa_alloc(&dma_heap_minors, &heap->minor, heap,
> >                      XA_LIMIT(0, NUM_HEAP_MINORS - 1), GFP_KERNEL);
> >       if (ret < 0) {
> >               pr_err("dma_heap: Unable to get minor number for heap\n");
> > @@ -236,7 +241,7 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
> >       }
> >
> >       /* Create device */
> > -     heap->heap_devt = MKDEV(MAJOR(dma_heap_devt), minor);
> > +     heap->heap_devt = MKDEV(MAJOR(dma_heap_devt), heap->minor);
> >
> >       cdev_init(&heap->heap_cdev, &dma_heap_fops);
> >       ret = cdev_add(&heap->heap_cdev, heap->heap_devt, 1);
> > @@ -280,12 +285,37 @@ struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info)
> >   err2:
> >       cdev_del(&heap->heap_cdev);
> >   err1:
> > -     xa_erase(&dma_heap_minors, minor);
> > +     xa_erase(&dma_heap_minors, heap->minor);
> >   err0:
> >       kfree(heap);
> >       return err_ret;
> >   }
> >
> > +static void dma_heap_release(struct kref *ref)
> > +{
> > +     struct dma_heap *heap = container_of(ref, struct dma_heap, refcount);
> > +
> > +     /* Note, we already holding the heap_list_lock here */
> > +     list_del(&heap->list);
> > +
> > +     device_destroy(dma_heap_class, heap->heap_devt);
> > +     cdev_del(&heap->heap_cdev);
> > +     xa_erase(&dma_heap_minors, heap->minor);
>
> You can just use MINOR(heap->heap_devt) here instead.
>
Got it, thanks.

> > +
> > +     kfree(heap);
> > +}
> > +
> > +void dma_heap_put(struct dma_heap *h)
> > +{
> > +     /*
> > +      * Take the heap_list_lock now to avoid racing with code
> > +      * scanning the list and then taking a kref.
> > +      */
>
> This is usually considered a bad idea since it makes the kref approach
> superfluous.
>
> There are multiple possibilities how handle this, the most common one is
> to use kref_get_unless_zero() in your list traversal code and ignore the
> entry when that fails.
>
> Alternatively you could use kref_put_mutex() instead. This gives you the
> same functionality as this here, but as far as I know it's normally only
> used in a couple of special cases.
>
Ok, I'll move this mutex acquisition to dma_heap_release so that it
guards just the list_del, and change dma_heap_find to use
kref_get_unless_zero. Thanks.

> > +     mutex_lock(&heap_list_lock);
> > +     kref_put(&h->refcount, dma_heap_release);
> > +     mutex_unlock(&heap_list_lock);
> > +}
> > +
> >   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 c7c29b724ad6..f3c678892c5c 100644
> > --- a/include/linux/dma-heap.h
> > +++ b/include/linux/dma-heap.h
> > @@ -64,4 +64,10 @@ const char *dma_heap_get_name(struct dma_heap *heap);
> >    */
> >   struct dma_heap *dma_heap_add(const struct dma_heap_export_info *exp_info);
> >
> > +/**
> > + * dma_heap_put - drops a reference to a dmabuf heap, potentially freeing it
> > + * @heap: the heap whose reference count to decrement
> > + */
>
> Please don't add kerneldoc to the definition, add it to the
> implementation of the function.
>
Will fix.




> Regards,
> Christian.
>
> > +void dma_heap_put(struct dma_heap *heap);
> > +
> >   #endif /* _DMA_HEAPS_H */
>