[PATCH 5/5] DONOTMERGE: dma-buf: Get rid of dma_fence_get_rcu_safe
Christian König
christian.koenig at amd.com
Thu Jun 10 06:51:16 UTC 2021
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.
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.
Regards,
Christian.
>
> Signed-off-by: Jason Ekstrand <jason at jlekstrand.net>
> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
> Cc: Christian König <christian.koenig at amd.com>
> Cc: Matthew Auld <matthew.auld at intel.com>
> Cc: Maarten Lankhorst <maarten.lankhorst at linux.intel.com>
> ---
> 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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