On Thu, Jun 10, 2021 at 1:51 AM Christian König
<christian.koenig@xxxxxxx> wrote:
>
> Am 09.06.21 um 23:29 schrieb Jason Ekstrand:
> > This helper existed to handle the weird corner-cases caused by using
> > SLAB_TYPESAFE_BY_RCU for backing dma_fence. Now that no one is using
> > that anymore (i915 was the only real user), dma_fence_get_rcu is
> > sufficient. The one slightly annoying thing we have to deal with here
> > is that dma_fence_get_rcu_safe did an rcu_dereference as well as a
> > SLAB_TYPESAFE_BY_RCU-safe dma_fence_get_rcu. This means each call site
> > ends up being 3 lines instead of 1.
>
> That's an outright NAK.
>
> The loop in dma_fence_get_rcu_safe is necessary because the underlying
> fence object can be replaced while taking the reference.
Right. I had missed a bit of that when I first read through it. I
see the need for the loop now. But there are some other tricky bits
in there besides just the loop.
> This is completely unrelated to SLAB_TYPESAFE_BY_RCU. See the
> dma_fence_chain usage for reference.
>
> What you can remove is the sequence number handling in dma-buf. That
> should make adding fences quite a bit quicker.
I'll look at that and try to understand what's going on there.
--Jason
> Regards,
> Christian.
>
> >
> > Signed-off-by: Jason Ekstrand <jason@xxxxxxxxxxxxxx>
> > Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> > Cc: Christian König <christian.koenig@xxxxxxx>
> > Cc: Matthew Auld <matthew.auld@xxxxxxxxx>
> > Cc: Maarten Lankhorst <maarten.lankhorst@xxxxxxxxxxxxxxx>
> > ---
> > drivers/dma-buf/dma-fence-chain.c | 8 ++--
> > drivers/dma-buf/dma-resv.c | 4 +-
> > drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c | 4 +-
> > drivers/gpu/drm/i915/i915_active.h | 4 +-
> > drivers/gpu/drm/i915/i915_vma.c | 4 +-
> > include/drm/drm_syncobj.h | 4 +-
> > include/linux/dma-fence.h | 50 -----------------------
> > include/linux/dma-resv.h | 4 +-
> > 8 files changed, 23 insertions(+), 59 deletions(-)
> >
> > diff --git a/drivers/dma-buf/dma-fence-chain.c b/drivers/dma-buf/dma-fence-chain.c
> > index 7d129e68ac701..46dfc7d94d8ed 100644
> > --- a/drivers/dma-buf/dma-fence-chain.c
> > +++ b/drivers/dma-buf/dma-fence-chain.c
> > @@ -15,15 +15,17 @@ static bool dma_fence_chain_enable_signaling(struct dma_fence *fence);
> > * dma_fence_chain_get_prev - use RCU to get a reference to the previous fence
> > * @chain: chain node to get the previous node from
> > *
> > - * Use dma_fence_get_rcu_safe to get a reference to the previous fence of the
> > - * chain node.
> > + * Use rcu_dereference and dma_fence_get_rcu to get a reference to the
> > + * previous fence of the chain node.
> > */
> > static struct dma_fence *dma_fence_chain_get_prev(struct dma_fence_chain *chain)
> > {
> > struct dma_fence *prev;
> >
> > rcu_read_lock();
> > - prev = dma_fence_get_rcu_safe(&chain->prev);
> > + prev = rcu_dereference(chain->prev);
> > + if (prev)
> > + prev = dma_fence_get_rcu(prev);
> > rcu_read_unlock();
> > return prev;
> > }
> > diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
> > index f26c71747d43a..cfe0db3cca292 100644
> > --- a/drivers/dma-buf/dma-resv.c
> > +++ b/drivers/dma-buf/dma-resv.c
> > @@ -376,7 +376,9 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
> > dst_list = NULL;
> > }
> >
> > - new = dma_fence_get_rcu_safe(&src->fence_excl);
> > + new = rcu_dereference(src->fence_excl);
> > + if (new)
> > + new = dma_fence_get_rcu(new);
> > rcu_read_unlock();
> >
> > src_list = dma_resv_shared_list(dst);
> > diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
> > index 72d9b92b17547..0aeb6117f3893 100644
> > --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
> > +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
> > @@ -161,7 +161,9 @@ int amdgpu_fence_emit(struct amdgpu_ring *ring, struct dma_fence **f,
> > struct dma_fence *old;
> >
> > rcu_read_lock();
> > - old = dma_fence_get_rcu_safe(ptr);
> > + old = rcu_dereference(*ptr);
> > + if (old)
> > + old = dma_fence_get_rcu(old);
> > rcu_read_unlock();
> >
> > if (old) {
> > diff --git a/drivers/gpu/drm/i915/i915_active.h b/drivers/gpu/drm/i915/i915_active.h
> > index d0feda68b874f..bd89cfc806ca5 100644
> > --- a/drivers/gpu/drm/i915/i915_active.h
> > +++ b/drivers/gpu/drm/i915/i915_active.h
> > @@ -103,7 +103,9 @@ i915_active_fence_get(struct i915_active_fence *active)
> > struct dma_fence *fence;
> >
> > rcu_read_lock();
> > - fence = dma_fence_get_rcu_safe(&active->fence);
> > + fence = rcu_dereference(active->fence);
> > + if (fence)
> > + fence = dma_fence_get_rcu(fence);
> > rcu_read_unlock();
> >
> > return fence;
> > diff --git a/drivers/gpu/drm/i915/i915_vma.c b/drivers/gpu/drm/i915/i915_vma.c
> > index 0f227f28b2802..ed0388d99197e 100644
> > --- a/drivers/gpu/drm/i915/i915_vma.c
> > +++ b/drivers/gpu/drm/i915/i915_vma.c
> > @@ -351,7 +351,9 @@ int i915_vma_wait_for_bind(struct i915_vma *vma)
> > struct dma_fence *fence;
> >
> > rcu_read_lock();
> > - fence = dma_fence_get_rcu_safe(&vma->active.excl.fence);
> > + fence = rcu_dereference(vma->active.excl.fence);
> > + if (fence)
> > + fence = dma_fence_get_rcu(fence);
> > rcu_read_unlock();
> > if (fence) {
> > err = dma_fence_wait(fence, MAX_SCHEDULE_TIMEOUT);
> > diff --git a/include/drm/drm_syncobj.h b/include/drm/drm_syncobj.h
> > index 6cf7243a1dc5e..6c45d52988bcc 100644
> > --- a/include/drm/drm_syncobj.h
> > +++ b/include/drm/drm_syncobj.h
> > @@ -105,7 +105,9 @@ drm_syncobj_fence_get(struct drm_syncobj *syncobj)
> > struct dma_fence *fence;
> >
> > rcu_read_lock();
> > - fence = dma_fence_get_rcu_safe(&syncobj->fence);
> > + fence = rcu_dereference(syncobj->fence);
> > + if (fence)
> > + fence = dma_fence_get_rcu(syncobj->fence);
> > rcu_read_unlock();
> >
> > return fence;
> > diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h
> > index 6ffb4b2c63715..f4a2ab2b1ae46 100644
> > --- a/include/linux/dma-fence.h
> > +++ b/include/linux/dma-fence.h
> > @@ -307,56 +307,6 @@ static inline struct dma_fence *dma_fence_get_rcu(struct dma_fence *fence)
> > return NULL;
> > }
> >
> > -/**
> > - * dma_fence_get_rcu_safe - acquire a reference to an RCU tracked fence
> > - * @fencep: pointer to fence to increase refcount of
> > - *
> > - * Function returns NULL if no refcount could be obtained, or the fence.
> > - * This function handles acquiring a reference to a fence that may be
> > - * reallocated within the RCU grace period (such as with SLAB_TYPESAFE_BY_RCU),
> > - * so long as the caller is using RCU on the pointer to the fence.
> > - *
> > - * An alternative mechanism is to employ a seqlock to protect a bunch of
> > - * fences, such as used by struct dma_resv. When using a seqlock,
> > - * the seqlock must be taken before and checked after a reference to the
> > - * fence is acquired (as shown here).
> > - *
> > - * The caller is required to hold the RCU read lock.
> > - */
> > -static inline struct dma_fence *
> > -dma_fence_get_rcu_safe(struct dma_fence __rcu **fencep)
> > -{
> > - do {
> > - struct dma_fence *fence;
> > -
> > - fence = rcu_dereference(*fencep);
> > - if (!fence)
> > - return NULL;
> > -
> > - if (!dma_fence_get_rcu(fence))
> > - continue;
> > -
> > - /* The atomic_inc_not_zero() inside dma_fence_get_rcu()
> > - * provides a full memory barrier upon success (such as now).
> > - * This is paired with the write barrier from assigning
> > - * to the __rcu protected fence pointer so that if that
> > - * pointer still matches the current fence, we know we
> > - * have successfully acquire a reference to it. If it no
> > - * longer matches, we are holding a reference to some other
> > - * reallocated pointer. This is possible if the allocator
> > - * is using a freelist like SLAB_TYPESAFE_BY_RCU where the
> > - * fence remains valid for the RCU grace period, but it
> > - * may be reallocated. When using such allocators, we are
> > - * responsible for ensuring the reference we get is to
> > - * the right fence, as below.
> > - */
> > - if (fence == rcu_access_pointer(*fencep))
> > - return rcu_pointer_handoff(fence);
> > -
> > - dma_fence_put(fence);
> > - } while (1);
> > -}
> > -
> > #ifdef CONFIG_LOCKDEP
> > bool dma_fence_begin_signalling(void);
> > void dma_fence_end_signalling(bool cookie);
> > diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
> > index 562b885cf9c3d..a38c021f379af 100644
> > --- a/include/linux/dma-resv.h
> > +++ b/include/linux/dma-resv.h
> > @@ -248,7 +248,9 @@ dma_resv_get_excl_unlocked(struct dma_resv *obj)
> > return NULL;
> >
> > rcu_read_lock();
> > - fence = dma_fence_get_rcu_safe(&obj->fence_excl);
> > + fence = rcu_dereference(obj->fence_excl);
> > + if (fence)
> > + fence = dma_fence_get_rcu(fence);
> > rcu_read_unlock();
> >
> > return fence;
>
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