Re: [PATCH 1/4] dma-buf: change DMA-buf locking convention v2

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On Tue, Oct 22, 2019 at 12:01:30PM +0200, Daniel Vetter wrote:
> On Mon, Oct 21, 2019 at 01:15:21PM +0200, Christian König wrote:
> > This patch is a stripped down version of the locking changes
> > necessary to support dynamic DMA-buf handling.
> > 
> > It adds a dynamic flag for both importers as well as exporters
> > so that drivers can choose if they want the reservation object
> > locked or unlocked during mapping of attachments.
> > 
> > For compatibility between drivers we cache the DMA-buf mapping
> > during attaching an importer as soon as exporter/importer
> > disagree on the dynamic handling.
> > 
> > This change has gone through a lengthy discussion on dri-devel
> > and other mailing lists with at least 3-4 different attempts and
> > dead-ends until we settled on this solution. Please refer to the
> > mailing lists archives for full background on the history of
> > this change.

Two more refinments below, I hit send a notch too early.
> 
> I kinda hoped for a real write-up of why we ended up here, not a "please
> read the last year or so of dri-devel" ...
> 
> So here's what I think we need to minimally mention, pls add:
> 
> <cut>
> Issues and solutions we considered:
> 
> - We can't change all existing drivers, and existing drivers have strong
>   opinions about which locks they're holding while calling
>   dma_buf_attachment_map/unmap. The solution to avoid this was to move the

Maybe add here "Exporters also have strong opinions about which locks they
can acquire in their ->map/unmap callbacks, levaing no room for change."

>   actual map/unmap out from this call, into the attach/detach callbacks,
>   and cache the mapping. This works because drivers don't call
>   attach/detach from deep within their code callchains (like deep in
>   memory management code called from cs/execbuf ioctl), but directly from
>   the fd2handle implementation.
> 
> - The caching has some troubles on some soc drivers, which set other modes
>   than DMA_BIDIRECTIONAL. We can't have 2 incompatible mappings, and we
>   can't re-create the mapping at _map time due to the above locking fun.
>   We very carefuly step around that by only caching at attach time if the
>   dynamic mode between importer/expoert mismatches.
> 
> - There's been quite some discussion on dma-buf mappings which need active
>   cache management, which would all break down when caching, plus we don't
>   have explicit flush operations on the attachment side. The solution to
>   this was to shrug and keep the current discrepancy between what the
>   dma-buf docs claim and what implementations do, with the hope that the
>   begin/end_cpu_access hooks are good enough and that all necessary
>   flushing to keep device mappings consistent will be done there.
> </cut>
> > 
> > v2: cleanup set_name merge, improve kerneldoc
> > 
> > Signed-off-by: Christian König <christian.koenig@xxxxxxx>
> > ---
> >  drivers/dma-buf/dma-buf.c | 102 +++++++++++++++++++++++++++++++++-----
> >  include/linux/dma-buf.h   |  57 +++++++++++++++++++--
> >  2 files changed, 143 insertions(+), 16 deletions(-)
> > 
> > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > index 433d91d710e4..753be84b5fd6 100644
> > --- a/drivers/dma-buf/dma-buf.c
> > +++ b/drivers/dma-buf/dma-buf.c
> > @@ -45,10 +45,10 @@ static char *dmabuffs_dname(struct dentry *dentry, char *buffer, int buflen)
> >  	size_t ret = 0;
> >  
> >  	dmabuf = dentry->d_fsdata;
> > -	mutex_lock(&dmabuf->lock);
> > +	dma_resv_lock(dmabuf->resv, NULL);
> >  	if (dmabuf->name)
> >  		ret = strlcpy(name, dmabuf->name, DMA_BUF_NAME_LEN);
> > -	mutex_unlock(&dmabuf->lock);
> > +	dma_resv_unlock(dmabuf->resv);
> >  
> >  	return dynamic_dname(dentry, buffer, buflen, "/%s:%s",
> >  			     dentry->d_name.name, ret > 0 ? name : "");
> > @@ -334,7 +334,7 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
> >  	if (IS_ERR(name))
> >  		return PTR_ERR(name);
> >  
> > -	mutex_lock(&dmabuf->lock);
> > +	dma_resv_lock(dmabuf->resv, NULL);
> >  	if (!list_empty(&dmabuf->attachments)) {
> >  		ret = -EBUSY;
> >  		kfree(name);
> > @@ -344,7 +344,7 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
> >  	dmabuf->name = name;
> >  
> >  out_unlock:
> > -	mutex_unlock(&dmabuf->lock);
> > +	dma_resv_unlock(dmabuf->resv);
> >  	return ret;
> >  }
> >  
> > @@ -403,10 +403,10 @@ static void dma_buf_show_fdinfo(struct seq_file *m, struct file *file)
> >  	/* Don't count the temporary reference taken inside procfs seq_show */
> >  	seq_printf(m, "count:\t%ld\n", file_count(dmabuf->file) - 1);
> >  	seq_printf(m, "exp_name:\t%s\n", dmabuf->exp_name);
> > -	mutex_lock(&dmabuf->lock);
> > +	dma_resv_lock(dmabuf->resv, NULL);
> >  	if (dmabuf->name)
> >  		seq_printf(m, "name:\t%s\n", dmabuf->name);
> > -	mutex_unlock(&dmabuf->lock);
> > +	dma_resv_unlock(dmabuf->resv);
> >  }
> >  
> >  static const struct file_operations dma_buf_fops = {
> > @@ -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);
> > +		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);
> 
> Almost tripped me up until I noticed your check for the buf/exporter here.
> Nice check!
> 
> Maybe we could do an
> 
> 	else
> 		lockdep_assert_not_held()
> 
> here? Open-coded ofc (or put it into drm_utils.h, I don't want to hold up
> this patch any longer, then move it to lockdep.h later on), since
> currently doesn't exist. Just to _really_ enforce the documented contract
> here.

On 2nd thought, probably better to do this as a follow up 2 patch series,
first patch adding the lockdep_assert_not_held to lockdep.h, 2nd one
adding the else here and in the unmap function below. Otherwise we're
going to be stuck in another bikeshed here.

Cheers, Daniel


> 

> > +
> >  	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);
> 
> You've switched dmabuf->name to be protected by dma_resv too, so this
> needs to be moved up above the seq_printf which prints ->name.
> 
> >  		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..bcc0f4d0b678 100644
> > --- a/include/linux/dma-buf.h
> > +++ b/include/linux/dma-buf.h
> > @@ -42,6 +42,18 @@ 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
> 
> s/ure/sure/
> 
> > +	 * callbacks are always called without the dma_resv object locked.
> > +	 * Mutual exclusive with @cache_sgt_mapping.
> > +	 */
> > +	bool dynamic_mapping;
> > +
> >  	/**
> >  	 * @attach:
> >  	 *
> > @@ -109,6 +121,9 @@ 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,
> > @@ -267,7 +282,8 @@ struct dma_buf_ops {
> >   * struct dma_buf - shared buffer object
> >   * @size: size of the buffer
> >   * @file: file pointer used for sharing buffers across, and for refcounting.
> > - * @attachments: list of dma_buf_attachment that denotes all devices attached.
> > + * @attachments: list of dma_buf_attachment that denotes all devices attached,
> > + *               protected by dma_resv lock.
> >   * @ops: dma_buf_ops associated with this buffer object.
> >   * @lock: used internally to serialize list manipulation, attach/detach and
> >   *        vmap/unmap, and accesses to name
> 
> @name in dma_buf needs to be updated to mention that it's protected by
> dma_resv lock.
> 
> > @@ -323,10 +339,12 @@ struct dma_buf {
> >   * struct dma_buf_attachment - holds device-buffer attachment data
> >   * @dmabuf: buffer for this attachment.
> >   * @dev: device attached to the buffer.
> > - * @node: list of dma_buf_attachment.
> > + * @node: list of dma_buf_attachment, protected by dma_resv lock of the dmabuf.
> >   * @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 +361,7 @@ struct dma_buf_attachment {
> >  	struct list_head node;
> >  	struct sg_table *sgt;
> >  	enum dma_data_direction dir;
> > +	bool dynamic_mapping;
> >  	void *priv;
> >  };
> >  
> > @@ -394,10 +413,39 @@ 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.
> 
> Maybe added "... for the map/unmap callbacks, ..." since we don't
> guarantee to hold the lock for all callbacks. You clarify this already
> below.
> 
> > + */
> > +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 +457,7 @@ 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,
> 
> With the nits all addressed:
> 
> Reviewed-by: Daniel Vetter <daniel.vetter@xxxxxxxx>
> 
> > -- 
> > 2.17.1
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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