Re: [PATCH 2/4] dma-buf: lock the reservation object during (un)map_dma_buf v2

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Jun 25, 2018 at 10:22:31AM +0200, Daniel Vetter wrote:
> On Fri, Jun 22, 2018 at 04:11:01PM +0200, Christian König wrote:
> > First step towards unpinned DMA buf operation.
> > 
> > I've checked the DRM drivers to potential locking of the reservation
> > object, but essentially we need to audit all implementations of the
> > dma_buf _ops for this to work.
> > 
> > v2: reordered
> > 
> > Signed-off-by: Christian König <christian.koenig@amd.com>
> 
> Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>

Ok I did review drivers a bit, but apparently not well enough by far. i915
CI is unhappy:

https://intel-gfx-ci.01.org/tree/drm-tip/Patchwork_9400/fi-whl-u/igt@gem_mmap_gtt@basic-small-bo-tiledx.html

So yeah inserting that lock in there isn't the most trivial thing :-/

I kinda assume that other drivers will have similar issues, e.g. omapdrm's
use of dev->struct_mutex also very much looks like it'll result in a new
locking inversion.
-Daniel

> 
> > ---
> >  drivers/dma-buf/dma-buf.c | 9 ++++++---
> >  include/linux/dma-buf.h   | 4 ++++
> >  2 files changed, 10 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > index dc94e76e2e2a..49f23b791eb8 100644
> > --- a/drivers/dma-buf/dma-buf.c
> > +++ b/drivers/dma-buf/dma-buf.c
> > @@ -665,7 +665,9 @@ struct sg_table *dma_buf_map_attachment(struct dma_buf_attachment *attach,
> >  	if (WARN_ON(!attach || !attach->dmabuf))
> >  		return ERR_PTR(-EINVAL);
> >  
> > -	sg_table = attach->dmabuf->ops->map_dma_buf(attach, direction);
> > +	reservation_object_lock(attach->dmabuf->resv, NULL);
> > +	sg_table = dma_buf_map_attachment_locked(attach, direction);
> > +	reservation_object_unlock(attach->dmabuf->resv);
> >  	if (!sg_table)
> >  		sg_table = ERR_PTR(-ENOMEM);
> >  
> > @@ -715,8 +717,9 @@ void dma_buf_unmap_attachment(struct dma_buf_attachment *attach,
> >  	if (WARN_ON(!attach || !attach->dmabuf || !sg_table))
> >  		return;
> >  
> > -	attach->dmabuf->ops->unmap_dma_buf(attach, sg_table,
> > -						direction);
> > +	reservation_object_lock(attach->dmabuf->resv, NULL);
> > +	dma_buf_unmap_attachment_locked(attach, sg_table, direction);
> > +	reservation_object_unlock(attach->dmabuf->resv);
> >  }
> >  EXPORT_SYMBOL_GPL(dma_buf_unmap_attachment);
> >  
> > diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h
> > index a25e754ae2f7..024658d1f22e 100644
> > --- a/include/linux/dma-buf.h
> > +++ b/include/linux/dma-buf.h
> > @@ -118,6 +118,8 @@ struct dma_buf_ops {
> >  	 * any other kind of sharing that the exporter might wish to make
> >  	 * available to buffer-users.
> >  	 *
> > +	 * This is called with the dmabuf->resv object locked.
> > +	 *
> >  	 * Returns:
> >  	 *
> >  	 * A &sg_table scatter list of or the backing storage of the DMA buffer,
> > @@ -138,6 +140,8 @@ struct dma_buf_ops {
> >  	 * It should also unpin the backing storage if this is the last mapping
> >  	 * of the DMA buffer, it the exporter supports backing storage
> >  	 * migration.
> > +	 *
> > +	 * This is called with the dmabuf->resv object locked.
> >  	 */
> >  	void (*unmap_dma_buf)(struct dma_buf_attachment *,
> >  			      struct sg_table *,
> > -- 
> > 2.14.1
> > 
> 
> -- 
> Daniel Vetter
> Software Engineer, Intel Corporation
> http://blog.ffwll.ch

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