在 2019/4/1 21:05, Christian König 写道:
> Am 01.04.19 um 04:54 schrieb Zhou, David(ChunMing):
>>
>>> -----Original Message-----
>>> From: amd-gfx <amd-gfx-bounces@xxxxxxxxxxxxxxxxxxxxx> On Behalf Of
>>> Christian K?nig
>>> Sent: Saturday, March 30, 2019 2:33 AM
>>> To: amd-gfx@xxxxxxxxxxxxxxxxxxxxx
>>> Subject: [PATCH] drm/amdgpu: fix old fence check in amdgpu_fence_emit
>>>
>>> We don't hold a reference to the old fence, so it can go away any
>>> time we are
>>> waiting for it to signal.
>>>
>>> Signed-off-by: Christian König <christian.koenig@xxxxxxx>
>>> ---
>>> drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c | 24 ++++++++++++++++-
>>> ------
>>> 1 file changed, 17 insertions(+), 7 deletions(-)
>>>
>>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
>>> b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
>>> index ee47c11e92ce..4dee2326b29c 100644
>>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
>>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_fence.c
>>> @@ -136,8 +136,9 @@ int amdgpu_fence_emit(struct amdgpu_ring *ring,
>>> struct dma_fence **f, {
>>> struct amdgpu_device *adev = ring->adev;
>>> struct amdgpu_fence *fence;
>>> - struct dma_fence *old, **ptr;
>>> + struct dma_fence __rcu **ptr;
>>> uint32_t seq;
>>> + int r;
>>>
>>> fence = kmem_cache_alloc(amdgpu_fence_slab, GFP_KERNEL);
>>> if (fence == NULL)
>>> @@ -153,15 +154,24 @@ int amdgpu_fence_emit(struct amdgpu_ring *ring,
>>> struct dma_fence **f,
>>> seq, flags | AMDGPU_FENCE_FLAG_INT);
>>>
>>> ptr = &ring->fence_drv.fences[seq & ring-
>>>> fence_drv.num_fences_mask];
>>> + if (unlikely(rcu_dereference_protected(*ptr, 1))) {
>> Isn't this line redundant with dma_fence_get_rcu_safe? I think it's
>> unnecessary.
>> Otherwise looks ok to me.
>
> The key point is lock()+dma_fence_get_rcu_safe(ptr)+unlock() is rather
> expensive for something which is really unlikely.
>
> So we check here if we already see the variable as NULL and if that is
> true, then we can just skip the whole expensive dance.
but that is most unlikely case, isn't it? That ptr is NULL seems only
when before first fence emitted.
-David
>
> Christian.
>
>>
>> -David
>>> + struct dma_fence *old;
>>> +
>>> + rcu_read_lock();
>>> + old = dma_fence_get_rcu_safe(ptr);
>>> + rcu_read_unlock();
>>> +
>>> + if (old) {
>>> + r = dma_fence_wait(old, false);
>>> + dma_fence_put(old);
>>> + if (r)
>>> + return r;
>>> + }
>>> + }
>>> +
>>> /* This function can't be called concurrently anyway, otherwise
>>> * emitting the fence would mess up the hardware ring buffer.
>>> */
>>> - old = rcu_dereference_protected(*ptr, 1);
>>> - if (old && !dma_fence_is_signaled(old)) {
>>> - DRM_INFO("rcu slot is busy\n");
>>> - dma_fence_wait(old, false);
>>> - }
>>> -
>>> rcu_assign_pointer(*ptr, dma_fence_get(&fence->base));
>>>
>>> *f = &fence->base;
>>> --
>>> 2.17.1
>>>
>>> _______________________________________________
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>>> amd-gfx@xxxxxxxxxxxxxxxxxxxxx
>>> https://lists.freedesktop.org/mailman/listinfo/amd-gfx
>
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