On Thu, Jun 10, 2021 at 3:59 PM Jason Ekstrand <jason@xxxxxxxxxxxxxx> wrote:
>
> 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.
I thought that's what the kref_get_unless_zero was for in
dma_fence_get_rcu? Otherwise I guess I'm not seeing why still have
dma_fence_get_rcu around, since that should either be a kref_get or
it's just unsafe to call it ...
> > 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.
Hm I thought the seqlock was to make sure we have a consistent set of
fences across exclusive and all shared slot. Not to protect against
the fence disappearing due to typesafe_by_rcu.
-Daniel
> --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;
> >
--
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
