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Message-ID: <71c435a6-ba43-5d26-e658-f55bddbb8b98@amd.com>
Date: Mon, 11 Sep 2023 11:48:46 +0200
From: Christian König <christian.koenig@....com>
To: Yong Wu <yong.wu@...iatek.com>, Rob Herring <robh+dt@...nel.org>,
Sumit Semwal <sumit.semwal@...aro.org>,
Matthias Brugger <matthias.bgg@...il.com>
Cc: Krzysztof Kozlowski <krzysztof.kozlowski+dt@...aro.org>,
Conor Dooley <conor+dt@...nel.org>,
Benjamin Gaignard <benjamin.gaignard@...labora.com>,
Brian Starkey <Brian.Starkey@....com>,
John Stultz <jstultz@...gle.com>, tjmercier@...gle.com,
AngeloGioacchino Del Regno
<angelogioacchino.delregno@...labora.com>,
devicetree@...r.kernel.org, linux-kernel@...r.kernel.org,
linux-media@...r.kernel.org, dri-devel@...ts.freedesktop.org,
linaro-mm-sig@...ts.linaro.org,
linux-arm-kernel@...ts.infradead.org,
linux-mediatek@...ts.infradead.org, jianjiao.zeng@...iatek.com,
kuohong.wang@...iatek.com
Subject: Re: [PATCH 2/9] dma-heap: Add proper kref handling on dma-buf heaps
Am 11.09.23 um 04:30 schrieb Yong Wu:
> From: John Stultz <jstultz@...gle.com>
>
> 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@...gle.com>
> Signed-off-by: T.J. Mercier <tjmercier@...gle.com>
> Signed-off-by: Yong Wu <yong.wu@...iatek.com>
> [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.
> +
> + 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.
> + 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.
Regards,
Christian.
> +void dma_heap_put(struct dma_heap *heap);
> +
> #endif /* _DMA_HEAPS_H */
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