Re: [PATCH 1/4] dma-buf: change DMA-buf locking convention

Daniel Vetter
2019-09-03T08:05:28Z

On Thu, Aug 29, 2019 at 04:29:14PM +0200, Christian König wrote:
> This patch is a stripped down version of the locking changes
> necessary to support dynamic DMA-buf handling.
>
> For compatibility we cache the DMA-buf mapping as soon as
> exporter/importer disagree on the dynamic handling.
>
> Signed-off-by: Christian König <christian.koenig@amd.com>
> ---
> drivers/dma-buf/dma-buf.c | 90 ++++++++++++++++++++++++++++++++++++---
> include/linux/dma-buf.h | 51 +++++++++++++++++++++-
> 2 files changed, 133 insertions(+), 8 deletions(-)
>
> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> index 433d91d710e4..65052d52602b 100644
> --- a/drivers/dma-buf/dma-buf.c
> +++ b/drivers/dma-buf/dma-buf.c
> @@ -525,6 +525,10 @@ struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info)
> return ERR_PTR(-EINVAL);
> }
>
> + if (WARN_ON(exp_info->ops->cache_sgt_mapping &&
> + exp_info->ops->dynamic_mapping))
> + return ERR_PTR(-EINVAL);
> +
> if (!try_module_get(exp_info->owner))
> return ERR_PTR(-ENOENT);
>
> @@ -645,10 +649,11 @@ void dma_buf_put(struct dma_buf *dmabuf)
> EXPORT_SYMBOL_GPL(dma_buf_put);
>
> /**
> - * dma_buf_attach - Add the device to dma_buf's attachments list; optionally,
> + * dma_buf_dynamic_attach - Add the device to dma_buf's attachments list; optionally,
> * calls attach() of dma_buf_ops to allow device-specific attach functionality
> - * @dmabuf: [in] buffer to attach device to.
> - * @dev: [in] device to be attached.
> + * @dmabuf: [in] buffer to attach device to.
> + * @dev: [in] device to be attached.
> + * @dynamic_mapping: [in] calling convention for map/unmap
> *
> * Returns struct dma_buf_attachment pointer for this attachment. Attachments
> * must be cleaned up by calling dma_buf_detach().
> @@ -662,8 +667,9 @@ EXPORT_SYMBOL_GPL(dma_buf_put);
> * accessible to @dev, and cannot be moved to a more suitable place. This is
> * indicated with the error code -EBUSY.
> */
> -struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
> - struct device *dev)
> +struct dma_buf_attachment *
> +dma_buf_dynamic_attach(struct dma_buf *dmabuf, struct device *dev,
> + bool dynamic_mapping)
> {
> struct dma_buf_attachment *attach;
> int ret;
> @@ -677,6 +683,7 @@ struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
>
> attach->dev = dev;
> attach->dmabuf = dmabuf;
> + attach->dynamic_mapping = dynamic_mapping;
>
> mutex_lock(&dmabuf->lock);
>
> @@ -685,16 +692,64 @@ struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
> if (ret)
> goto err_attach;
> }
> + dma_resv_lock(dmabuf->resv, NULL);
> list_add(&attach->node, &dmabuf->attachments);
> + dma_resv_unlock(dmabuf->resv);
>
> mutex_unlock(&dmabuf->lock);
>
> + /* When either the importer or the exporter can't handle dynamic
> + * mappings we cache the mapping here to avoid issues with the
> + * reservation object lock.
> + */
> + if (dma_buf_attachment_is_dynamic(attach) !=
> + dma_buf_is_dynamic(dmabuf)) {
> + struct sg_table *sgt;
> +
> + if (dma_buf_is_dynamic(attach->dmabuf))
> + dma_resv_lock(attach->dmabuf->resv, NULL);
> +
> + sgt = dmabuf->ops->map_dma_buf(attach, DMA_BIDIRECTIONAL);

Now we're back to enforcing DMA_BIDI, which works nicely around the
locking pain, but apparently upsets the arm-soc folks who want to control
this better. That's why your previous version moved the caching into
map/unmap_sg, which resurrected the locking hydra.

I think we're going a bit in circles here, and I don't have a good idea
either :-/
-Daniel

> + if (!sgt)
> + sgt = ERR_PTR(-ENOMEM);
> + if (IS_ERR(sgt)) {
> + ret = PTR_ERR(sgt);
> + goto err_unlock;
> + }
> + if (dma_buf_is_dynamic(attach->dmabuf))
> + dma_resv_unlock(attach->dmabuf->resv);
> + attach->sgt = sgt;
> + attach->dir = DMA_BIDIRECTIONAL;
> + }
> +
> return attach;
>
> err_attach:
> kfree(attach);
> mutex_unlock(&dmabuf->lock);
> return ERR_PTR(ret);
> +
> +err_unlock:
> + if (dma_buf_is_dynamic(attach->dmabuf))
> + dma_resv_unlock(attach->dmabuf->resv);
> +
> + dma_buf_detach(dmabuf, attach);
> + return ERR_PTR(ret);
> +}
> +EXPORT_SYMBOL_GPL(dma_buf_dynamic_attach);
> +
> +/**
> + * dma_buf_attach - Wrapper for dma_buf_dynamic_attach
> + * @dmabuf: [in] buffer to attach device to.
> + * @dev: [in] device to be attached.
> + *
> + * Wrapper to call dma_buf_dynamic_attach() for drivers which still use a static
> + * mapping.
> + */
> +struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
> + struct device *dev)
> +{
> + return dma_buf_dynamic_attach(dmabuf, dev, false);
> }
> EXPORT_SYMBOL_GPL(dma_buf_attach);
>
> @@ -711,11 +766,20 @@ void dma_buf_detach(struct dma_buf *dmabuf, struct dma_buf_attachment *attach)
> if (WARN_ON(!dmabuf || !attach))
> return;
>
> - if (attach->sgt)
> + if (attach->sgt) {
> + if (dma_buf_is_dynamic(attach->dmabuf))
> + dma_resv_lock(attach->dmabuf->resv, NULL);
> +
> dmabuf->ops->unmap_dma_buf(attach, attach->sgt, attach->dir);
>
> + if (dma_buf_is_dynamic(attach->dmabuf))
> + dma_resv_unlock(attach->dmabuf->resv);
> + }
> +
> mutex_lock(&dmabuf->lock);
> + dma_resv_lock(dmabuf->resv, NULL);
> list_del(&attach->node);
> + dma_resv_unlock(dmabuf->resv);
> if (dmabuf->ops->detach)
> dmabuf->ops->detach(dmabuf, attach);
>
> @@ -749,6 +813,9 @@ struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
> if (WARN_ON(!attach || !attach->dmabuf))
> return ERR_PTR(-EINVAL);
>
> + if (dma_buf_attachment_is_dynamic(attach))
> + dma_resv_assert_held(attach->dmabuf->resv);
> +
> if (attach->sgt) {
> /*
> * Two mappings with different directions for the same
> @@ -761,6 +828,9 @@ struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
> return attach->sgt;
> }
>
> + if (dma_buf_is_dynamic(attach->dmabuf))
> + dma_resv_assert_held(attach->dmabuf->resv);
> +
> sg_table = attach->dmabuf->ops->map_dma_buf(attach, direction);
> if (!sg_table)
> sg_table = ERR_PTR(-ENOMEM);
> @@ -793,9 +863,15 @@ void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,
> if (WARN_ON(!attach || !attach->dmabuf || !sg_table))
> return;
>
> + if (dma_buf_attachment_is_dynamic(attach))
> + dma_resv_assert_held(attach->dmabuf->resv);
> +
> if (attach->sgt == sg_table)
> return;
>
> + if (dma_buf_is_dynamic(attach->dmabuf))
> + dma_resv_assert_held(attach->dmabuf->resv);
> +
> attach->dmabuf->ops->unmap_dma_buf(attach, sg_table, direction);
> }
> EXPORT_SYMBOL_GPL(dma_buf_unmap_attachment);
> @@ -1219,10 +1295,12 @@ static int dma_buf_debug_show(struct seq_file *s, void *unused)
> seq_puts(s, "\tAttached Devices:\n");
> attach_count = 0;
>
> + dma_resv_lock(buf_obj->resv, NULL);
> list_for_each_entry(attach_obj, &buf_obj->attachments, node) {
> seq_printf(s, "\t%s\n", dev_name(attach_obj->dev));
> attach_count++;
> }
> + dma_resv_unlock(buf_obj->resv);
>
> seq_printf(s, "Total %d devices attached\n\n",
> attach_count);
> diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> index ec212cb27fdc..a8f8b2b812fd 100644
> --- a/include/linux/dma-buf.h
> +++ b/include/linux/dma-buf.h
> @@ -42,6 +42,17 @@ struct dma_buf_ops {
> */
> bool cache_sgt_mapping;
>
> + /**
> + * @dynamic_mapping:
> + *
> + * If true the framework makes sure that the map/unmap_dma_buf
> + * callbacks are always called with the dma_resv object locked.
> + *
> + * If false the framework makes ure that the map/unmap_dma_buf
> + * callbacks are always called without the dma_resv object locked.
> + */
> + bool dynamic_mapping;
> +
> /**
> * @attach:
> *
> @@ -109,6 +120,9 @@ struct dma_buf_ops {
> * any other kind of sharing that the exporter might wish to make
> * available to buffer-users.
> *
> + * This is always called with the dmabuf->resv object locked when
> + * the dynamic_mapping flag is true.
> + *
> * Returns:
> *
> * A &sg_table scatter list of or the backing storage of the DMA buffer,
> @@ -327,6 +341,8 @@ struct dma_buf {
> * @sgt: cached mapping.
> * @dir: direction of cached mapping.
> * @priv: exporter specific attachment data.
> + * @dynamic_mapping: true if dma_buf_map/unmap_attachment() is called with the
> + * dma_resv lock held.
> *
> * This structure holds the attachment information between the dma_buf buffer
> * and its user device(s). The list contains one attachment struct per device
> @@ -343,6 +359,7 @@ struct dma_buf_attachment {
> struct list_head node;
> struct sg_table *sgt;
> enum dma_data_direction dir;
> + bool dynamic_mapping;
> void *priv;
> };
>
> @@ -394,10 +411,39 @@ static inline void get_dma_buf(struct dma_buf *dmabuf)
> get_file(dmabuf->file);
> }
>
> +/**
> + * dma_buf_is_dynamic - check if a DMA-buf uses dynamic mappings.
> + * @dmabuf: the DMA-buf to check
> + *
> + * Returns true if a DMA-buf exporter wants to be called with the dma_resv
> + * locked, false if it doesn't wants to be called with the lock held.
> + */
> +static inline bool dma_buf_is_dynamic(struct dma_buf *dmabuf)
> +{
> + return dmabuf->ops->dynamic_mapping;
> +}
> +
> +/**
> + * dma_buf_attachment_is_dynamic - check if a DMA-buf attachment uses dynamic
> + * mappinsg
> + * @attach: the DMA-buf attachment to check
> + *
> + * Returns true if a DMA-buf importer wants to call the map/unmap functions with
> + * the dma_resv lock held.
> + */
> +static inline bool
> +dma_buf_attachment_is_dynamic(struct dma_buf_attachment *attach)
> +{
> + return attach->dynamic_mapping;
> +}
> +
> struct dma_buf_attachment *dma_buf_attach(struct dma_buf *dmabuf,
> - struct device *dev);
> + struct device *dev);
> +struct dma_buf_attachment *
> +dma_buf_dynamic_attach(struct dma_buf *dmabuf, struct device *dev,
> + bool dynamic_mapping);
> void dma_buf_detach(struct dma_buf *dmabuf,
> - struct dma_buf_attachment *dmabuf_attach);
> + struct dma_buf_attachment *attach);
>
> struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info);
>
> @@ -409,6 +455,7 @@ struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *,
> enum dma_data_direction);
> void dma_buf_unmap_attachment(struct dma_buf_attachment *, struct sg_table *,
> enum dma_data_direction);
> +void dma_buf_move_notify(struct dma_buf *dma_buf);
> int dma_buf_begin_cpu_access(struct dma_buf *dma_buf,
> enum dma_data_direction dir);
> int dma_buf_end_cpu_access(struct dma_buf *dma_buf,
> --
> 2.17.1
>

--
Daniel Vetter
Software Engineer, Intel Corporation