[Intel-gfx] [PATCH 1/4] dma-buf: change DMA-buf locking convention v2
Daniel Vetter
daniel at ffwll.ch
Tue Oct 22 10:04:41 UTC 2019
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 at amd.com>
> > ---
> > 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 at ffwll.ch>
>
> > --
> > 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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