Re: [PATCH v2 06/18] dma-buf: Use sequence counter with associated wound/wait mutex

[Daniel Vetter <daniel@ffwll.ch>]

On Mon, Jun 08, 2020 at 02:57:17AM +0200, Ahmed S. Darwish wrote:
> A sequence counter write side critical section must be protected by some
> form of locking to serialize writers. If the serialization primitive is
> not disabling preemption implicitly, preemption has to be explicitly
> disabled before entering the sequence counter write side critical
> section.
> 
> The dma-buf reservation subsystem uses plain sequence counters to manage
> updates to reservations. Writer serialization is accomplished through a
> wound/wait mutex.
> 
> Acquiring a wound/wait mutex does not disable preemption, so this needs
> to be done manually before and after the write side critical section.
> 
> Use the newly-added seqcount_ww_mutex_t instead:
> 
>   - It associates the ww_mutex with the sequence count, which enables
>     lockdep to validate that the write side critical section is properly
>     serialized.
> 
>   - It removes the need to explicitly add preempt_disable/enable()
>     around the write side critical section because the write_begin/end()
>     functions for this new data type automatically do this.
> 
> If lockdep is disabled this ww_mutex lock association is compiled out
> and has neither storage size nor runtime overhead.
> 
> Signed-off-by: Ahmed S. Darwish <a.darwish@linutronix.de>

I'm not seeing the patch that adds the seqcount ww_mutex glue and not
quite motivated enough to grab it from lore, so someone else needs to
check the details. Just

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>

for merging through whatever tree/branch makes sense from me.
-Daniel

> ---
>  drivers/dma-buf/dma-resv.c                       | 8 +-------
>  drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c | 2 --
>  include/linux/dma-resv.h                         | 2 +-
>  3 files changed, 2 insertions(+), 10 deletions(-)
> 
> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
> index 590ce7ad60a0..3aba2b2bfc48 100644
> --- a/drivers/dma-buf/dma-resv.c
> +++ b/drivers/dma-buf/dma-resv.c
> @@ -128,7 +128,7 @@ subsys_initcall(dma_resv_lockdep);
>  void dma_resv_init(struct dma_resv *obj)
>  {
>  	ww_mutex_init(&obj->lock, &reservation_ww_class);
> -	seqcount_init(&obj->seq);
> +	seqcount_ww_mutex_init(&obj->seq, &obj->lock);
>  
>  	RCU_INIT_POINTER(obj->fence, NULL);
>  	RCU_INIT_POINTER(obj->fence_excl, NULL);
> @@ -259,7 +259,6 @@ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
>  	fobj = dma_resv_get_list(obj);
>  	count = fobj->shared_count;
>  
> -	preempt_disable();
>  	write_seqcount_begin(&obj->seq);
>  
>  	for (i = 0; i < count; ++i) {
> @@ -281,7 +280,6 @@ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
>  	smp_store_mb(fobj->shared_count, count);
>  
>  	write_seqcount_end(&obj->seq);
> -	preempt_enable();
>  	dma_fence_put(old);
>  }
>  EXPORT_SYMBOL(dma_resv_add_shared_fence);
> @@ -308,14 +306,12 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
>  	if (fence)
>  		dma_fence_get(fence);
>  
> -	preempt_disable();
>  	write_seqcount_begin(&obj->seq);
>  	/* write_seqcount_begin provides the necessary memory barrier */
>  	RCU_INIT_POINTER(obj->fence_excl, fence);
>  	if (old)
>  		old->shared_count = 0;
>  	write_seqcount_end(&obj->seq);
> -	preempt_enable();
>  
>  	/* inplace update, no shared fences */
>  	while (i--)
> @@ -393,13 +389,11 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
>  	src_list = dma_resv_get_list(dst);
>  	old = dma_resv_get_excl(dst);
>  
> -	preempt_disable();
>  	write_seqcount_begin(&dst->seq);
>  	/* write_seqcount_begin provides the necessary memory barrier */
>  	RCU_INIT_POINTER(dst->fence_excl, new);
>  	RCU_INIT_POINTER(dst->fence, dst_list);
>  	write_seqcount_end(&dst->seq);
> -	preempt_enable();
>  
>  	dma_resv_list_free(src_list);
>  	dma_fence_put(old);
> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
> index 6a5b91d23fd9..c71c0bb6ce26 100644
> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
> @@ -258,11 +258,9 @@ static int amdgpu_amdkfd_remove_eviction_fence(struct amdgpu_bo *bo,
>  	new->shared_count = k;
>  
>  	/* Install the new fence list, seqcount provides the barriers */
> -	preempt_disable();
>  	write_seqcount_begin(&resv->seq);
>  	RCU_INIT_POINTER(resv->fence, new);
>  	write_seqcount_end(&resv->seq);
> -	preempt_enable();
>  
>  	/* Drop the references to the removed fences or move them to ef_list */
>  	for (i = j, k = 0; i < old->shared_count; ++i) {
> diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
> index a6538ae7d93f..d44a77e8a7e3 100644
> --- a/include/linux/dma-resv.h
> +++ b/include/linux/dma-resv.h
> @@ -69,7 +69,7 @@ struct dma_resv_list {
>   */
>  struct dma_resv {
>  	struct ww_mutex lock;
> -	seqcount_t seq;
> +	seqcount_ww_mutex_t seq;
>  
>  	struct dma_fence __rcu *fence_excl;
>  	struct dma_resv_list __rcu *fence;
> -- 
> 2.20.1
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch
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