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