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@xxxxxxxxxxxxxxxxxx>
> Cc: Chris Wilson <chris@xxxxxxxxxxxxxxxxxx>
I said I needed to go lie down, that proves it.
Cc: Christian König <christian.koenig@xxxxxxx>
> Cc: Daniel Vetter <daniel.vetter@xxxxxxxx>
> ---
> 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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