[Intel-gfx] [PATCH] dma-buf: Restore seqlock around dma_resv updates

Koenig, Christian Christian.Koenig at amd.com
Thu Aug 15 07:30:22 UTC 2019


Am 14.08.19 um 20:26 schrieb Chris Wilson:
> Quoting Chris Wilson (2019-08-14 19:24:01)
>> This reverts
>> 67c97fb79a7f ("dma-buf: add reservation_object_fences helper")
>> dd7a7d1ff2f1 ("drm/i915: use new reservation_object_fences helper")
>> 0e1d8083bddb ("dma-buf: further relax reservation_object_add_shared_fence")
>> 5d344f58da76 ("dma-buf: nuke reservation_object seq number")
>>
>> The scenario that defeats simply grabbing a set of shared/exclusive
>> fences and using them blissfully under RCU is that any of those fences
>> may be reallocated by a SLAB_TYPESAFE_BY_RCU fence slab cache. In this
>> scenario, while keeping the rcu_read_lock we need to establish that no
>> fence was changed in the dma_resv after a read (or full) memory barrier.
>>
>> Signed-off-by: Chris Wilson <chris at chris-wilson.co.uk>

Acked-by: Christian König <christian.koenig at amd.com>

>> Cc: Chris Wilson <chris at chris-wilson.co.uk>
> I said I needed to go lie down, that proves it.
>
> Cc: Christian König <christian.koenig at amd.com>
>
>> Cc: Daniel Vetter <daniel.vetter at ffwll.ch>
>> ---
>>   drivers/dma-buf/dma-buf.c                     |  31 ++++-
>>   drivers/dma-buf/dma-resv.c                    | 109 ++++++++++++-----
>>   .../gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c  |   7 +-
>>   drivers/gpu/drm/i915/gem/i915_gem_busy.c      |  24 ++--
>>   include/linux/dma-resv.h                      | 113 ++++++++----------
>>   5 files changed, 175 insertions(+), 109 deletions(-)
>>
>> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
>> index b3400d6524ab..433d91d710e4 100644
>> --- a/drivers/dma-buf/dma-buf.c
>> +++ b/drivers/dma-buf/dma-buf.c
>> @@ -199,7 +199,7 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
>>          struct dma_resv_list *fobj;
>>          struct dma_fence *fence_excl;
>>          __poll_t events;
>> -       unsigned shared_count;
>> +       unsigned shared_count, seq;
>>   
>>          dmabuf = file->private_data;
>>          if (!dmabuf || !dmabuf->resv)
>> @@ -213,8 +213,21 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll)
>>          if (!events)
>>                  return 0;
>>   
>> +retry:
>> +       seq = read_seqcount_begin(&resv->seq);
>>          rcu_read_lock();
>> -       dma_resv_fences(resv, &fence_excl, &fobj, &shared_count);
>> +
>> +       fobj = rcu_dereference(resv->fence);
>> +       if (fobj)
>> +               shared_count = fobj->shared_count;
>> +       else
>> +               shared_count = 0;
>> +       fence_excl = rcu_dereference(resv->fence_excl);
>> +       if (read_seqcount_retry(&resv->seq, seq)) {
>> +               rcu_read_unlock();
>> +               goto retry;
>> +       }
>> +
>>          if (fence_excl && (!(events & EPOLLOUT) || shared_count == 0)) {
>>                  struct dma_buf_poll_cb_t *dcb = &dmabuf->cb_excl;
>>                  __poll_t pevents = EPOLLIN;
>> @@ -1144,6 +1157,7 @@ static int dma_buf_debug_show(struct seq_file *s, void *unused)
>>          struct dma_resv *robj;
>>          struct dma_resv_list *fobj;
>>          struct dma_fence *fence;
>> +       unsigned seq;
>>          int count = 0, attach_count, shared_count, i;
>>          size_t size = 0;
>>   
>> @@ -1174,9 +1188,16 @@ static int dma_buf_debug_show(struct seq_file *s, void *unused)
>>                                  buf_obj->name ?: "");
>>   
>>                  robj = buf_obj->resv;
>> -               rcu_read_lock();
>> -               dma_resv_fences(robj, &fence, &fobj, &shared_count);
>> -               rcu_read_unlock();
>> +               while (true) {
>> +                       seq = read_seqcount_begin(&robj->seq);
>> +                       rcu_read_lock();
>> +                       fobj = rcu_dereference(robj->fence);
>> +                       shared_count = fobj ? fobj->shared_count : 0;
>> +                       fence = rcu_dereference(robj->fence_excl);
>> +                       if (!read_seqcount_retry(&robj->seq, seq))
>> +                               break;
>> +                       rcu_read_unlock();
>> +               }
>>   
>>                  if (fence)
>>                          seq_printf(s, "\tExclusive fence: %s %s %ssignalled\n",
>> diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c
>> index f5142683c851..42a8f3f11681 100644
>> --- a/drivers/dma-buf/dma-resv.c
>> +++ b/drivers/dma-buf/dma-resv.c
>> @@ -49,6 +49,12 @@
>>   DEFINE_WD_CLASS(reservation_ww_class);
>>   EXPORT_SYMBOL(reservation_ww_class);
>>   
>> +struct lock_class_key reservation_seqcount_class;
>> +EXPORT_SYMBOL(reservation_seqcount_class);
>> +
>> +const char reservation_seqcount_string[] = "reservation_seqcount";
>> +EXPORT_SYMBOL(reservation_seqcount_string);
>> +
>>   /**
>>    * dma_resv_list_alloc - allocate fence list
>>    * @shared_max: number of fences we need space for
>> @@ -96,6 +102,9 @@ static void dma_resv_list_free(struct dma_resv_list *list)
>>   void dma_resv_init(struct dma_resv *obj)
>>   {
>>          ww_mutex_init(&obj->lock, &reservation_ww_class);
>> +
>> +       __seqcount_init(&obj->seq, reservation_seqcount_string,
>> +                       &reservation_seqcount_class);
>>          RCU_INIT_POINTER(obj->fence, NULL);
>>          RCU_INIT_POINTER(obj->fence_excl, NULL);
>>   }
>> @@ -225,6 +234,9 @@ 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) {
>>   
>>                  old = rcu_dereference_protected(fobj->shared[i],
>> @@ -242,6 +254,9 @@ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence)
>>          RCU_INIT_POINTER(fobj->shared[i], fence);
>>          /* pointer update must be visible before we extend the shared_count */
>>          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);
>> @@ -269,10 +284,12 @@ void dma_resv_add_excl_fence(struct dma_resv *obj, struct dma_fence *fence)
>>                  dma_fence_get(fence);
>>   
>>          preempt_disable();
>> -       rcu_assign_pointer(obj->fence_excl, fence);
>> -       /* pointer update must be visible before we modify the shared_count */
>> +       write_seqcount_begin(&obj->seq);
>> +       /* write_seqcount_begin provides the necessary memory barrier */
>> +       RCU_INIT_POINTER(obj->fence_excl, fence);
>>          if (old)
>> -               smp_store_mb(old->shared_count, 0);
>> +               old->shared_count = 0;
>> +       write_seqcount_end(&obj->seq);
>>          preempt_enable();
>>   
>>          /* inplace update, no shared fences */
>> @@ -295,15 +312,17 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
>>   {
>>          struct dma_resv_list *src_list, *dst_list;
>>          struct dma_fence *old, *new;
>> -       unsigned int i, shared_count;
>> +       unsigned i;
>>   
>>          dma_resv_assert_held(dst);
>>   
>>          rcu_read_lock();
>> +       src_list = rcu_dereference(src->fence);
>>   
>>   retry:
>> -       dma_resv_fences(src, &new, &src_list, &shared_count);
>> -       if (shared_count) {
>> +       if (src_list) {
>> +               unsigned shared_count = src_list->shared_count;
>> +
>>                  rcu_read_unlock();
>>   
>>                  dst_list = dma_resv_list_alloc(shared_count);
>> @@ -311,14 +330,14 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
>>                          return -ENOMEM;
>>   
>>                  rcu_read_lock();
>> -               dma_resv_fences(src, &new, &src_list, &shared_count);
>> -               if (!src_list || shared_count > dst_list->shared_max) {
>> +               src_list = rcu_dereference(src->fence);
>> +               if (!src_list || src_list->shared_count > shared_count) {
>>                          kfree(dst_list);
>>                          goto retry;
>>                  }
>>   
>>                  dst_list->shared_count = 0;
>> -               for (i = 0; i < shared_count; ++i) {
>> +               for (i = 0; i < src_list->shared_count; ++i) {
>>                          struct dma_fence *fence;
>>   
>>                          fence = rcu_dereference(src_list->shared[i]);
>> @@ -328,6 +347,7 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
>>   
>>                          if (!dma_fence_get_rcu(fence)) {
>>                                  dma_resv_list_free(dst_list);
>> +                               src_list = rcu_dereference(src->fence);
>>                                  goto retry;
>>                          }
>>   
>> @@ -342,18 +362,18 @@ int dma_resv_copy_fences(struct dma_resv *dst, struct dma_resv *src)
>>                  dst_list = NULL;
>>          }
>>   
>> -       if (new && !dma_fence_get_rcu(new)) {
>> -               dma_resv_list_free(dst_list);
>> -               goto retry;
>> -       }
>> +       new = dma_fence_get_rcu_safe(&src->fence_excl);
>>          rcu_read_unlock();
>>   
>>          src_list = dma_resv_get_list(dst);
>>          old = dma_resv_get_excl(dst);
>>   
>>          preempt_disable();
>> -       rcu_assign_pointer(dst->fence_excl, new);
>> -       rcu_assign_pointer(dst->fence, dst_list);
>> +       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);
>> @@ -388,18 +408,19 @@ int dma_resv_get_fences_rcu(struct dma_resv *obj,
>>   
>>          do {
>>                  struct dma_resv_list *fobj;
>> -               unsigned int i;
>> +               unsigned int i, seq;
>>                  size_t sz = 0;
>>   
>> -               i = 0;
>> +               shared_count = i = 0;
>>   
>>                  rcu_read_lock();
>> -               dma_resv_fences(obj, &fence_excl, &fobj,
>> -                                         &shared_count);
>> +               seq = read_seqcount_begin(&obj->seq);
>>   
>> +               fence_excl = rcu_dereference(obj->fence_excl);
>>                  if (fence_excl && !dma_fence_get_rcu(fence_excl))
>>                          goto unlock;
>>   
>> +               fobj = rcu_dereference(obj->fence);
>>                  if (fobj)
>>                          sz += sizeof(*shared) * fobj->shared_max;
>>   
>> @@ -427,6 +448,7 @@ int dma_resv_get_fences_rcu(struct dma_resv *obj,
>>                                  break;
>>                          }
>>                          shared = nshared;
>> +                       shared_count = fobj ? fobj->shared_count : 0;
>>                          for (i = 0; i < shared_count; ++i) {
>>                                  shared[i] = rcu_dereference(fobj->shared[i]);
>>                                  if (!dma_fence_get_rcu(shared[i]))
>> @@ -434,7 +456,7 @@ int dma_resv_get_fences_rcu(struct dma_resv *obj,
>>                          }
>>                  }
>>   
>> -               if (i != shared_count) {
>> +               if (i != shared_count || read_seqcount_retry(&obj->seq, seq)) {
>>                          while (i--)
>>                                  dma_fence_put(shared[i]);
>>                          dma_fence_put(fence_excl);
>> @@ -478,17 +500,18 @@ long dma_resv_wait_timeout_rcu(struct dma_resv *obj,
>>                                 bool wait_all, bool intr,
>>                                 unsigned long timeout)
>>   {
>> -       struct dma_resv_list *fobj;
>>          struct dma_fence *fence;
>> -       unsigned shared_count;
>> +       unsigned seq, shared_count;
>>          long ret = timeout ? timeout : 1;
>>          int i;
>>   
>>   retry:
>> +       shared_count = 0;
>> +       seq = read_seqcount_begin(&obj->seq);
>>          rcu_read_lock();
>>          i = -1;
>>   
>> -       dma_resv_fences(obj, &fence, &fobj, &shared_count);
>> +       fence = rcu_dereference(obj->fence_excl);
>>          if (fence && !test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) {
>>                  if (!dma_fence_get_rcu(fence))
>>                          goto unlock_retry;
>> @@ -503,6 +526,11 @@ long dma_resv_wait_timeout_rcu(struct dma_resv *obj,
>>          }
>>   
>>          if (wait_all) {
>> +               struct dma_resv_list *fobj = rcu_dereference(obj->fence);
>> +
>> +               if (fobj)
>> +                       shared_count = fobj->shared_count;
>> +
>>                  for (i = 0; !fence && i < shared_count; ++i) {
>>                          struct dma_fence *lfence = rcu_dereference(fobj->shared[i]);
>>   
>> @@ -525,6 +553,11 @@ long dma_resv_wait_timeout_rcu(struct dma_resv *obj,
>>   
>>          rcu_read_unlock();
>>          if (fence) {
>> +               if (read_seqcount_retry(&obj->seq, seq)) {
>> +                       dma_fence_put(fence);
>> +                       goto retry;
>> +               }
>> +
>>                  ret = dma_fence_wait_timeout(fence, intr, ret);
>>                  dma_fence_put(fence);
>>                  if (ret > 0 && wait_all && (i + 1 < shared_count))
>> @@ -567,19 +600,23 @@ static inline int dma_resv_test_signaled_single(struct dma_fence *passed_fence)
>>    */
>>   bool dma_resv_test_signaled_rcu(struct dma_resv *obj, bool test_all)
>>   {
>> -       struct dma_resv_list *fobj;
>> -       struct dma_fence *fence_excl;
>> -       unsigned shared_count;
>> +       unsigned seq, shared_count;
>>          int ret;
>>   
>>          rcu_read_lock();
>>   retry:
>>          ret = true;
>> +       shared_count = 0;
>> +       seq = read_seqcount_begin(&obj->seq);
>>   
>> -       dma_resv_fences(obj, &fence_excl, &fobj, &shared_count);
>>          if (test_all) {
>>                  unsigned i;
>>   
>> +               struct dma_resv_list *fobj = rcu_dereference(obj->fence);
>> +
>> +               if (fobj)
>> +                       shared_count = fobj->shared_count;
>> +
>>                  for (i = 0; i < shared_count; ++i) {
>>                          struct dma_fence *fence = rcu_dereference(fobj->shared[i]);
>>   
>> @@ -589,14 +626,24 @@ bool dma_resv_test_signaled_rcu(struct dma_resv *obj, bool test_all)
>>                          else if (!ret)
>>                                  break;
>>                  }
>> -       }
>>   
>> -       if (!shared_count && fence_excl) {
>> -               ret = dma_resv_test_signaled_single(fence_excl);
>> -               if (ret < 0)
>> +               if (read_seqcount_retry(&obj->seq, seq))
>>                          goto retry;
>>          }
>>   
>> +       if (!shared_count) {
>> +               struct dma_fence *fence_excl = rcu_dereference(obj->fence_excl);
>> +
>> +               if (fence_excl) {
>> +                       ret = dma_resv_test_signaled_single(fence_excl);
>> +                       if (ret < 0)
>> +                               goto retry;
>> +
>> +                       if (read_seqcount_retry(&obj->seq, seq))
>> +                               goto retry;
>> +               }
>> +       }
>> +
>>          rcu_read_unlock();
>>          return ret;
>>   }
>> diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
>> index bc4ec6b20a87..76e3516484e7 100644
>> --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
>> +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c
>> @@ -251,7 +251,12 @@ static int amdgpu_amdkfd_remove_eviction_fence(struct amdgpu_bo *bo,
>>          new->shared_max = old->shared_max;
>>          new->shared_count = k;
>>   
>> -       rcu_assign_pointer(resv->fence, new);
>> +       /* 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/drivers/gpu/drm/i915/gem/i915_gem_busy.c b/drivers/gpu/drm/i915/gem/i915_gem_busy.c
>> index a2aff1d8290e..3d4f5775a4ba 100644
>> --- a/drivers/gpu/drm/i915/gem/i915_gem_busy.c
>> +++ b/drivers/gpu/drm/i915/gem/i915_gem_busy.c
>> @@ -83,8 +83,7 @@ i915_gem_busy_ioctl(struct drm_device *dev, void *data,
>>          struct drm_i915_gem_busy *args = data;
>>          struct drm_i915_gem_object *obj;
>>          struct dma_resv_list *list;
>> -       unsigned int i, shared_count;
>> -       struct dma_fence *excl;
>> +       unsigned int seq;
>>          int err;
>>   
>>          err = -ENOENT;
>> @@ -110,18 +109,29 @@ i915_gem_busy_ioctl(struct drm_device *dev, void *data,
>>           * to report the overall busyness. This is what the wait-ioctl does.
>>           *
>>           */
>> -       dma_resv_fences(obj->base.resv, &excl, &list, &shared_count);
>> +retry:
>> +       seq = raw_read_seqcount(&obj->base.resv->seq);
>>   
>>          /* Translate the exclusive fence to the READ *and* WRITE engine */
>> -       args->busy = busy_check_writer(excl);
>> +       args->busy =
>> +               busy_check_writer(rcu_dereference(obj->base.resv->fence_excl));
>>   
>>          /* Translate shared fences to READ set of engines */
>> -       for (i = 0; i < shared_count; ++i) {
>> -               struct dma_fence *fence = rcu_dereference(list->shared[i]);
>> +       list = rcu_dereference(obj->base.resv->fence);
>> +       if (list) {
>> +               unsigned int shared_count = list->shared_count, i;
>>   
>> -               args->busy |= busy_check_reader(fence);
>> +               for (i = 0; i < shared_count; ++i) {
>> +                       struct dma_fence *fence =
>> +                               rcu_dereference(list->shared[i]);
>> +
>> +                       args->busy |= busy_check_reader(fence);
>> +               }
>>          }
>>   
>> +       if (args->busy && read_seqcount_retry(&obj->base.resv->seq, seq))
>> +               goto retry;
>> +
>>          err = 0;
>>   out:
>>          rcu_read_unlock();
>> diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
>> index 38f2802afabb..ee50d10f052b 100644
>> --- a/include/linux/dma-resv.h
>> +++ b/include/linux/dma-resv.h
>> @@ -46,6 +46,8 @@
>>   #include <linux/rcupdate.h>
>>   
>>   extern struct ww_class reservation_ww_class;
>> +extern struct lock_class_key reservation_seqcount_class;
>> +extern const char reservation_seqcount_string[];
>>   
>>   /**
>>    * struct dma_resv_list - a list of shared fences
>> @@ -69,6 +71,7 @@ struct dma_resv_list {
>>    */
>>   struct dma_resv {
>>          struct ww_mutex lock;
>> +       seqcount_t seq;
>>   
>>          struct dma_fence __rcu *fence_excl;
>>          struct dma_resv_list __rcu *fence;
>> @@ -77,24 +80,6 @@ struct dma_resv {
>>   #define dma_resv_held(obj) lockdep_is_held(&(obj)->lock.base)
>>   #define dma_resv_assert_held(obj) lockdep_assert_held(&(obj)->lock.base)
>>   
>> -/**
>> - * dma_resv_get_excl - get the reservation object's
>> - * exclusive fence, with update-side lock held
>> - * @obj: the reservation object
>> - *
>> - * Returns the exclusive fence (if any).  Does NOT take a
>> - * reference. Writers must hold obj->lock, readers may only
>> - * hold a RCU read side lock.
>> - *
>> - * RETURNS
>> - * The exclusive fence or NULL
>> - */
>> -static inline struct dma_fence *dma_resv_get_excl(struct dma_resv *obj)
>> -{
>> -       return rcu_dereference_protected(obj->fence_excl,
>> -                                        dma_resv_held(obj));
>> -}
>> -
>>   /**
>>    * dma_resv_get_list - get the reservation object's
>>    * shared fence list, with update-side lock held
>> @@ -109,53 +94,6 @@ static inline struct dma_resv_list *dma_resv_get_list(struct dma_resv *obj)
>>                                           dma_resv_held(obj));
>>   }
>>   
>> -/**
>> - * dma_resv_fences - read consistent fence pointers
>> - * @obj: reservation object where we get the fences from
>> - * @excl: pointer for the exclusive fence
>> - * @list: pointer for the shared fence list
>> - *
>> - * Make sure we have a consisten exclusive fence and shared fence list.
>> - * Must be called with rcu read side lock held.
>> - */
>> -static inline void dma_resv_fences(struct dma_resv *obj,
>> -                                  struct dma_fence **excl,
>> -                                  struct dma_resv_list **list,
>> -                                  u32 *shared_count)
>> -{
>> -       do {
>> -               *excl = rcu_dereference(obj->fence_excl);
>> -               *list = rcu_dereference(obj->fence);
>> -               *shared_count = *list ? (*list)->shared_count : 0;
>> -               smp_rmb(); /* See dma_resv_add_excl_fence */
>> -       } while (rcu_access_pointer(obj->fence_excl) != *excl);
>> -}
>> -
>> -/**
>> - * dma_resv_get_excl_rcu - get the reservation object's
>> - * exclusive fence, without lock held.
>> - * @obj: the reservation object
>> - *
>> - * If there is an exclusive fence, this atomically increments it's
>> - * reference count and returns it.
>> - *
>> - * RETURNS
>> - * The exclusive fence or NULL if none
>> - */
>> -static inline struct dma_fence *dma_resv_get_excl_rcu(struct dma_resv *obj)
>> -{
>> -       struct dma_fence *fence;
>> -
>> -       if (!rcu_access_pointer(obj->fence_excl))
>> -               return NULL;
>> -
>> -       rcu_read_lock();
>> -       fence = dma_fence_get_rcu_safe(&obj->fence_excl);
>> -       rcu_read_unlock();
>> -
>> -       return fence;
>> -}
>> -
>>   /**
>>    * dma_resv_lock - lock the reservation object
>>    * @obj: the reservation object
>> @@ -290,6 +228,51 @@ static inline void dma_resv_unlock(struct dma_resv *obj)
>>          ww_mutex_unlock(&obj->lock);
>>   }
>>   
>> +/**
>> + * dma_resv_get_excl - get the reservation object's
>> + * exclusive fence, with update-side lock held
>> + * @obj: the reservation object
>> + *
>> + * Returns the exclusive fence (if any).  Does NOT take a
>> + * reference. Writers must hold obj->lock, readers may only
>> + * hold a RCU read side lock.
>> + *
>> + * RETURNS
>> + * The exclusive fence or NULL
>> + */
>> +static inline struct dma_fence *
>> +dma_resv_get_excl(struct dma_resv *obj)
>> +{
>> +       return rcu_dereference_protected(obj->fence_excl,
>> +                                        dma_resv_held(obj));
>> +}
>> +
>> +/**
>> + * dma_resv_get_excl_rcu - get the reservation object's
>> + * exclusive fence, without lock held.
>> + * @obj: the reservation object
>> + *
>> + * If there is an exclusive fence, this atomically increments it's
>> + * reference count and returns it.
>> + *
>> + * RETURNS
>> + * The exclusive fence or NULL if none
>> + */
>> +static inline struct dma_fence *
>> +dma_resv_get_excl_rcu(struct dma_resv *obj)
>> +{
>> +       struct dma_fence *fence;
>> +
>> +       if (!rcu_access_pointer(obj->fence_excl))
>> +               return NULL;
>> +
>> +       rcu_read_lock();
>> +       fence = dma_fence_get_rcu_safe(&obj->fence_excl);
>> +       rcu_read_unlock();
>> +
>> +       return fence;
>> +}
>> +
>>   void dma_resv_init(struct dma_resv *obj);
>>   void dma_resv_fini(struct dma_resv *obj);
>>   int dma_resv_reserve_shared(struct dma_resv *obj, unsigned int num_fences);
>> -- 
>> 2.23.0.rc1
>>



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