Explicitly document that code can't assume that shared fences signal after the exclusive fence.

Signed-off-by: Christian König christian.koenig@amd.com --- drivers/dma-buf/dma-resv.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/drivers/dma-buf/dma-resv.c b/drivers/dma-buf/dma-resv.c index f26c71747d43..4ab02b6c387a 100644 --- a/drivers/dma-buf/dma-resv.c +++ b/drivers/dma-buf/dma-resv.c @@ -235,7 +235,10 @@ EXPORT_SYMBOL(dma_resv_reset_shared_max); * @fence: the shared fence to add * * Add a fence to a shared slot, obj->lock must be held, and - * dma_resv_reserve_shared() has been called. + * dma_resv_reserve_shared() has been called. The shared fences can signal in + * any order and there is especially no guarantee that shared fences signal + * after the exclusive one. Code relying on any signaling order is broken and + * needs to be fixed. */ void dma_resv_add_shared_fence(struct dma_resv *obj, struct dma_fence *fence) {

Daniel pointed me towards this function and there are multiple obvious problems in the implementation.

First of all the retry loop is not working as intended. In general the retry makes only sense if you grab the reference first and then check the sequence values.

It's also good practice to keep the reference around when installing callbacks to fences you don't own.

Then we skipped checking the exclusive fence when shared fences were present.

And last the whole implementation was unnecessary complex and rather hard to understand which could lead to probably unexpected behavior of the IOCTL.

Fix all this by reworking the implementation from scratch.

Only mildly tested and needs a thoughtful review of the code.

v2: fix the reference counting as well

Signed-off-by: Christian König christian.koenig@amd.com --- drivers/dma-buf/dma-buf.c | 133 ++++++++++++++++---------------------- include/linux/dma-buf.h | 2 +- 2 files changed, 55 insertions(+), 80 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c index 511fe0d217a0..b67fbf4e3705 100644 --- a/drivers/dma-buf/dma-buf.c +++ b/drivers/dma-buf/dma-buf.c @@ -72,7 +72,7 @@ static void dma_buf_release(struct dentry *dentry) * If you hit this BUG() it means someone dropped their ref to the * dma-buf while still having pending operation to the buffer. */ - BUG_ON(dmabuf->cb_shared.active || dmabuf->cb_excl.active); + BUG_ON(dmabuf->cb_in.active || dmabuf->cb_out.active);

dmabuf->ops->release(dmabuf);

@@ -202,16 +202,20 @@ static void dma_buf_poll_cb(struct dma_fence *fence, struct dma_fence_cb *cb) wake_up_locked_poll(dcb->poll, dcb->active); dcb->active = 0; spin_unlock_irqrestore(&dcb->poll->lock, flags); + dma_fence_put(fence); }

static __poll_t dma_buf_poll(struct file *file, poll_table *poll) { + struct dma_buf_poll_cb_t *dcb; struct dma_buf *dmabuf; struct dma_resv *resv; struct dma_resv_list *fobj; struct dma_fence *fence_excl; - __poll_t events; unsigned shared_count, seq; + struct dma_fence *fence; + __poll_t events; + int r, i;

dmabuf = file->private_data; if (!dmabuf || !dmabuf->resv) @@ -225,99 +229,70 @@ static __poll_t dma_buf_poll(struct file *file, poll_table *poll) if (!events) return 0;

+ dcb = events & EPOLLOUT ? &dmabuf->cb_out : &dmabuf->cb_in; + + /* Only queue a new one if we are not still waiting for the old one */ + spin_lock_irq(&dmabuf->poll.lock); + if (dcb->active) + events = 0; + else + dcb->active = events; + spin_unlock_irq(&dmabuf->poll.lock); + if (!events) + return 0; + retry: seq = read_seqcount_begin(&resv->seq); rcu_read_lock();

fobj = rcu_dereference(resv->fence); - if (fobj) + if (fobj && events & EPOLLOUT) shared_count = fobj->shared_count; else shared_count = 0; - fence_excl = dma_resv_excl_fence(resv); - 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; - - if (shared_count == 0) - pevents |= EPOLLOUT; - - spin_lock_irq(&dmabuf->poll.lock); - if (dcb->active) { - dcb->active |= pevents; - events &= ~pevents; - } else - dcb->active = pevents; - spin_unlock_irq(&dmabuf->poll.lock); - - if (events & pevents) { - if (!dma_fence_get_rcu(fence_excl)) { - /* force a recheck */ - events &= ~pevents; - dma_buf_poll_cb(NULL, &dcb->cb); - } else if (!dma_fence_add_callback(fence_excl, &dcb->cb, - dma_buf_poll_cb)) { - events &= ~pevents; - dma_fence_put(fence_excl); - } else { - /* - * No callback queued, wake up any additional - * waiters. - */ - dma_fence_put(fence_excl); - dma_buf_poll_cb(NULL, &dcb->cb); - } + for (i = 0; i < shared_count; ++i) { + fence = rcu_dereference(fobj->shared[i]); + fence = dma_fence_get_rcu(fence); + if (!fence || read_seqcount_retry(&resv->seq, seq)) { + /* Concurrent modify detected, force re-check */ + dma_fence_put(fence); + rcu_read_unlock(); + goto retry; } - }

- if ((events & EPOLLOUT) && shared_count > 0) { - struct dma_buf_poll_cb_t *dcb = &dmabuf->cb_shared; - int i; - - /* Only queue a new callback if no event has fired yet */ - spin_lock_irq(&dmabuf->poll.lock); - if (dcb->active) - events &= ~EPOLLOUT; - else - dcb->active = EPOLLOUT; - spin_unlock_irq(&dmabuf->poll.lock); - - if (!(events & EPOLLOUT)) + r = dma_fence_add_callback(fence, &dcb->cb, dma_buf_poll_cb); + if (!r) { + /* Callback queued */ + events = 0; goto out; + } + dma_fence_put(fence); + }

- for (i = 0; i < shared_count; ++i) { - struct dma_fence *fence = rcu_dereference(fobj->shared[i]); - - if (!dma_fence_get_rcu(fence)) { - /* - * fence refcount dropped to zero, this means - * that fobj has been freed - * - * call dma_buf_poll_cb and force a recheck! - */ - events &= ~EPOLLOUT; - dma_buf_poll_cb(NULL, &dcb->cb); - break; - } - if (!dma_fence_add_callback(fence, &dcb->cb, - dma_buf_poll_cb)) { - dma_fence_put(fence); - events &= ~EPOLLOUT; - break; - } + fence = dma_resv_excl_fence(resv); + if (fence) { + fence = dma_fence_get_rcu(fence); + if (!fence || read_seqcount_retry(&resv->seq, seq)) { + /* Concurrent modify detected, force re-check */ dma_fence_put(fence); + rcu_read_unlock(); + goto retry; + }

- /* No callback queued, wake up any additional waiters. */ - if (i == shared_count) - dma_buf_poll_cb(NULL, &dcb->cb); + r = dma_fence_add_callback(fence, &dcb->cb, dma_buf_poll_cb); + if (!r) { + /* Callback queued */ + events = 0; + goto out; + } + dma_fence_put(fence_excl); }

+ /* No callback queued, wake up any additional waiters. */ + dma_buf_poll_cb(NULL, &dcb->cb); + out: rcu_read_unlock(); return events; @@ -562,8 +537,8 @@ struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info) dmabuf->owner = exp_info->owner; spin_lock_init(&dmabuf->name_lock); init_waitqueue_head(&dmabuf->poll); - dmabuf->cb_excl.poll = dmabuf->cb_shared.poll = &dmabuf->poll; - dmabuf->cb_excl.active = dmabuf->cb_shared.active = 0; + dmabuf->cb_in.poll = dmabuf->cb_out.poll = &dmabuf->poll; + dmabuf->cb_in.active = dmabuf->cb_out.active = 0;

if (!resv) { resv = (struct dma_resv *)&dmabuf[1]; diff --git a/include/linux/dma-buf.h b/include/linux/dma-buf.h index efdc56b9d95f..7e747ad54c81 100644 --- a/include/linux/dma-buf.h +++ b/include/linux/dma-buf.h @@ -329,7 +329,7 @@ struct dma_buf { wait_queue_head_t *poll;

__poll_t active; - } cb_excl, cb_shared; + } cb_in, cb_out; };

/**

Drivers also need to to sync to the exclusive fence when a shared one is present.

Signed-off-by: Christian König christian.koenig@amd.com --- drivers/gpu/drm/nouveau/nouveau_fence.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c b/drivers/gpu/drm/nouveau/nouveau_fence.c index 6b43918035df..05d0b3eb3690 100644 --- a/drivers/gpu/drm/nouveau/nouveau_fence.c +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c @@ -358,7 +358,7 @@ nouveau_fence_sync(struct nouveau_bo *nvbo, struct nouveau_channel *chan, bool e fobj = dma_resv_shared_list(resv); fence = dma_resv_excl_fence(resv);

- if (fence && (!exclusive || !fobj || !fobj->shared_count)) { + if (fence) { struct nouveau_channel *prev = NULL; bool must_wait = true;

We no longer need to add the exclusive fence as shared fence as welldrm/amdgpu: drop workaround for adding page table clears as shared fence

We no longer need to add the exclusive fence as shared fence as well..

Signed-off-by: Christian König christian.koenig@amd.com --- drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c | 8 +------- 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c index 0780e8d18992..d7baa207f391 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_gem.c @@ -207,7 +207,7 @@ static void amdgpu_gem_object_close(struct drm_gem_object *obj, INIT_LIST_HEAD(&duplicates);

tv.bo = &bo->tbo; - tv.num_shared = 2; + tv.num_shared = 1; list_add(&tv.head, &list);

amdgpu_vm_get_pd_bo(vm, &list, &vm_pd); @@ -226,12 +226,6 @@ static void amdgpu_gem_object_close(struct drm_gem_object *obj, if (!amdgpu_vm_ready(vm)) goto out_unlock;

- fence = dma_resv_excl_fence(bo->tbo.base.resv); - if (fence) { - amdgpu_bo_fence(bo, fence, true); - fence = NULL; - } - r = amdgpu_vm_clear_freed(adev, vm, &fence); if (r || !fence) goto out_unlock;