Re: [PATCH v1 13/25] dma-buf: Use sequence counter with associated wound/wait mutex

[Christian KÃnig]

Am 19.05.20 um 23:45 schrieb Ahmed S. Darwish:
> 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.

Mhm, is the dma_resv object the only user of this new seqcount_ww_mutex 
variant ?

If yes we are trying to get rid of this sequence counter for quite some 
time, so I would rather invest the additional time to finish this.

Regards,
Christian.

> Signed-off-by: Ahmed S. Darwish <a.darwish@xxxxxxxxxxxxx>
> ---
>   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 9dff792c9290..87fd32aae8f9 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;