[PATCH 1/2] dma-buf: make reservation_object_copy_fences rcu save

[Daniel Vetter]

On Thu, Sep 07, 2017 at 09:13:32AM +0200, Christian König wrote:
> Ping? David can you take a look?
> 
> Alex is on vacation and that is a rather important bug fix.

Works better when you cc Gustavo/Maarten/Sumits/Chris I think, for
anything dma-buf review needing.
-Daniel

> 
> Thanks,
> Christian.
> 
> Am 04.09.2017 um 21:02 schrieb Christian König:
> > From: Christian König <christian.koenig at amd.com>
> > 
> > Stop requiring that the src reservation object is locked for this operation.
> > 
> > Signed-off-by: Christian König <christian.koenig at amd.com>
> > ---
> >   drivers/dma-buf/reservation.c | 56 ++++++++++++++++++++++++++++++++-----------
> >   1 file changed, 42 insertions(+), 14 deletions(-)
> > 
> > diff --git a/drivers/dma-buf/reservation.c b/drivers/dma-buf/reservation.c
> > index dec3a81..b44d9d7 100644
> > --- a/drivers/dma-buf/reservation.c
> > +++ b/drivers/dma-buf/reservation.c
> > @@ -266,8 +266,7 @@ EXPORT_SYMBOL(reservation_object_add_excl_fence);
> >   * @dst: the destination reservation object
> >   * @src: the source reservation object
> >   *
> > -* Copy all fences from src to dst. Both src->lock as well as dst-lock must be
> > -* held.
> > +* Copy all fences from src to dst. dst-lock must be held.
> >   */
> >   int reservation_object_copy_fences(struct reservation_object *dst,
> >   				   struct reservation_object *src)
> > @@ -277,33 +276,62 @@ int reservation_object_copy_fences(struct reservation_object *dst,
> >   	size_t size;
> >   	unsigned i;
> > -	src_list = reservation_object_get_list(src);
> > +	rcu_read_lock();
> > +	src_list = rcu_dereference(src->fence);
> > +retry:
> >   	if (src_list) {
> > -		size = offsetof(typeof(*src_list),
> > -				shared[src_list->shared_count]);
> > +		unsigned shared_count = src_list->shared_count;
> > +
> > +		size = offsetof(typeof(*src_list), shared[shared_count]);
> > +		rcu_read_unlock();
> > +
> >   		dst_list = kmalloc(size, GFP_KERNEL);
> >   		if (!dst_list)
> >   			return -ENOMEM;
> > -		dst_list->shared_count = src_list->shared_count;
> > -		dst_list->shared_max = src_list->shared_count;
> > -		for (i = 0; i < src_list->shared_count; ++i)
> > -			dst_list->shared[i] =
> > -				dma_fence_get(src_list->shared[i]);
> > +		rcu_read_lock();
> > +		src_list = rcu_dereference(src->fence);
> > +		if (!src_list || src_list->shared_count > shared_count) {
> > +			kfree(dst_list);
> > +			goto retry;
> > +		}
> > +
> > +		dst_list->shared_count = 0;
> > +		dst_list->shared_max = shared_count;
> > +		for (i = 0; i < src_list->shared_count; ++i) {
> > +			struct dma_fence *fence;
> > +
> > +			fence = rcu_dereference(src_list->shared[i]);
> > +			if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT,
> > +				     &fence->flags))
> > +				continue;
> > +
> > +			if (!dma_fence_get_rcu(fence)) {
> > +				kfree(dst_list);
> > +				src_list = rcu_dereference(src->fence);
> > +				goto retry;
> > +			}
> > +
> > +			if (dma_fence_is_signaled(fence)) {
> > +				dma_fence_put(fence);
> > +				continue;
> > +			}
> > +
> > +			dst_list->shared[dst_list->shared_count++] = fence;
> > +		}
> >   	} else {
> >   		dst_list = NULL;
> >   	}
> > +	new = dma_fence_get_rcu_safe(&src->fence_excl);
> > +	rcu_read_unlock();
> > +
> >   	kfree(dst->staged);
> >   	dst->staged = NULL;
> >   	src_list = reservation_object_get_list(dst);
> > -
> >   	old = reservation_object_get_excl(dst);
> > -	new = reservation_object_get_excl(src);
> > -
> > -	dma_fence_get(new);
> >   	preempt_disable();
> >   	write_seqcount_begin(&dst->seq);
> 
> 
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-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch