[Linaro-mm-sig] Re: [PATCH 02/15] dma-buf: introduce user fence support

On 02/05/2022 17:37, Christian König wrote:

Start introducing a new part of the framework for handling user fences.

In strict opposition to normal fences user fences don't reliable finish in

reliably

a fixed amount of time and therefore can't be used as dependency in memory management.

Because of this user fences are marked with DMA_FENCE_FLAG_USER. Lockdep is checked that we can at least fault user pages when we check them for signaling.

This patch also adds a flag to dma_fence_get_stub() so that we can retrieve a signaled user fence. This can be used together with lockdep to test the handling in code path supporting user fences.

Signed-off-by: Christian König christian.koenig@amd.com

drivers/dma-buf/dma-fence-unwrap.c | 4 +-- drivers/dma-buf/dma-fence.c | 31 ++++++++++++------- drivers/dma-buf/st-dma-fence.c | 2 +- .../gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c | 2 +- drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c | 2 +- drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c | 2 +- drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c | 4 +-- drivers/gpu/drm/drm_syncobj.c | 10 +++--- include/linux/dma-fence.h | 17 +++++++++- 9 files changed, 49 insertions(+), 25 deletions(-)

diff --git a/drivers/dma-buf/dma-fence-unwrap.c b/drivers/dma-buf/dma-fence-unwrap.c index c9becc74896d..87ee2efced10 100644 --- a/drivers/dma-buf/dma-fence-unwrap.c +++ b/drivers/dma-buf/dma-fence-unwrap.c @@ -76,7 +76,7 @@ struct dma_fence *__dma_fence_merge(unsigned int num_fences, } if (count == 0)

• return dma_fence_get_stub();
  

• return dma_fence_get_stub(false);
  

if (count > INT_MAX) return NULL; @@ -129,7 +129,7 @@ struct dma_fence *__dma_fence_merge(unsigned int num_fences, } while (tmp); if (count == 0) {

• tmp = dma_fence_get_stub();
  

• tmp = dma_fence_get_stub(false);
  

  goto return_tmp; }

diff --git a/drivers/dma-buf/dma-fence.c b/drivers/dma-buf/dma-fence.c index 066400ed8841..52873f5eaeba 100644 --- a/drivers/dma-buf/dma-fence.c +++ b/drivers/dma-buf/dma-fence.c @@ -26,6 +26,7 @@ EXPORT_TRACEPOINT_SYMBOL(dma_fence_signaled); static DEFINE_SPINLOCK(dma_fence_stub_lock); static struct dma_fence dma_fence_stub; +static struct dma_fence dma_fence_user_stub; /*

• fence context counter: each execution context should have its own

@@ -123,24 +124,28 @@ static const struct dma_fence_ops dma_fence_stub_ops = { /**

• dma_fence_get_stub - return a signaled fence

• • @user: if true the returned fence is an user fence

• • Return a stub fence which is already signaled. The fence's
• • timestamp corresponds to the first time after boot this
• • function is called.

• • Return a stub fence which is already signaled. The fence's timestamp
• • corresponds to the first time after boot this function is called. If @user is
• • true an user fence is returned which can be used with lockdep to test user
• • fence saveness in a code path.
  */

-struct dma_fence *dma_fence_get_stub(void) +struct dma_fence *dma_fence_get_stub(bool user) {

• struct dma_fence *fence = user ? &dma_fence_stub : &dma_fence_user_stub;
• spin_lock(&dma_fence_stub_lock);

• if (!dma_fence_stub.ops) {

• dma_fence_init(&dma_fence_stub,
  

• 	       &dma_fence_stub_ops,
  

• 	       &dma_fence_stub_lock,
  

• if (!fence->ops) {

• dma_fence_init(fence, &dma_fence_stub_ops, &dma_fence_stub_lock,
         0, 0);
  

• dma_fence_signal_locked(&dma_fence_stub);
  

• if (user)
  

• 	set_bit(DMA_FENCE_FLAG_USER, &fence->flags);
  

• dma_fence_signal_locked(fence);
  

  } spin_unlock(&dma_fence_stub_lock);

• return dma_fence_get(&dma_fence_stub);

• return dma_fence_get(fence); } EXPORT_SYMBOL(dma_fence_get_stub);

@@ -497,8 +502,9 @@ dma_fence_wait_timeout(struct dma_fence *fence, bool intr, signed long timeout) return -EINVAL; might_sleep();

• __dma_fence_might_wait();

• if (test_bit(DMA_FENCE_FLAG_USER, &fence->flags))

• might_fault();
  

trace_dma_fence_wait_start(fence); if (fence->ops->wait) @@ -870,6 +876,9 @@ dma_fence_wait_any_timeout(struct dma_fence **fences, uint32_t count, for (i = 0; i < count; ++i) { struct dma_fence *fence = fences[i];

• if (test_bit(DMA_FENCE_FLAG_USER, &fence->flags))
  

• 	might_fault();
  

• cb[i].task = current; if (dma_fence_add_callback(fence, &cb[i].base, dma_fence_default_wait_cb)) {

diff --git a/drivers/dma-buf/st-dma-fence.c b/drivers/dma-buf/st-dma-fence.c index c8a12d7ad71a..50f757f75645 100644 --- a/drivers/dma-buf/st-dma-fence.c +++ b/drivers/dma-buf/st-dma-fence.c @@ -412,7 +412,7 @@ static int test_stub(void *arg) int i; for (i = 0; i < ARRAY_SIZE(f); i++) {

• f[i] = dma_fence_get_stub();
  

• f[i] = dma_fence_get_stub((i % 2) == 1);
  

  if (!dma_fence_is_signaled(f[i])) { pr_err("Obtained unsignaled stub fence!\n"); goto err;

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c index 64ac4f8f49be..541c59635c34 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_amdkfd_gpuvm.c @@ -263,7 +263,7 @@ static int amdgpu_amdkfd_remove_eviction_fence(struct amdgpu_bo *bo, /* TODO: Instead of block before we should use the fence of the page * table update and TLB flush here directly. */

• replacement = dma_fence_get_stub();

• replacement = dma_fence_get_stub(false); dma_resv_replace_fences(bo->tbo.base.resv, ef->base.context, replacement, DMA_RESV_USAGE_READ); dma_fence_put(replacement);

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c index a28b7947a034..95eeab527ca9 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_cs.c @@ -1399,7 +1399,7 @@ int amdgpu_cs_fence_to_handle_ioctl(struct drm_device *dev, void *data, return PTR_ERR(fence); if (!fence)

• fence = dma_fence_get_stub();
  

• fence = dma_fence_get_stub(false);
  

switch (info->in.what) { case AMDGPU_FENCE_TO_HANDLE_GET_SYNCOBJ: diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c index 7f33ae87cb41..73165f387f3b 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_ring.c @@ -193,7 +193,7 @@ int amdgpu_ring_init(struct amdgpu_device *adev, struct amdgpu_ring *ring, adev->rings[ring->idx] = ring; ring->num_hw_submission = sched_hw_submission; ring->sched_score = sched_score;

• ring->vmid_wait = dma_fence_get_stub();
  

• ring->vmid_wait = dma_fence_get_stub(false);
  

  r = amdgpu_fence_driver_init_ring(ring); if (r) return r;

diff --git a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c index 7e5cc8323329..e5c8e72a9485 100644 --- a/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c +++ b/drivers/gpu/drm/amd/amdgpu/amdgpu_vm.c @@ -1689,7 +1689,7 @@ int amdgpu_vm_bo_update_mapping(struct amdgpu_device *adev, } if (!unlocked && !dma_fence_is_signaled(vm->last_unlocked)) {

• struct dma_fence *tmp = dma_fence_get_stub();
  

• struct dma_fence *tmp = dma_fence_get_stub(false);
  

amdgpu_bo_fence(vm->root.bo, vm->last_unlocked, true); swap(vm->last_unlocked, tmp); @@ -2905,7 +2905,7 @@ int amdgpu_vm_init(struct amdgpu_device *adev, struct amdgpu_vm *vm) else vm->update_funcs = &amdgpu_vm_sdma_funcs; vm->last_update = NULL;

• vm->last_unlocked = dma_fence_get_stub();

• vm->last_unlocked = dma_fence_get_stub(false);

mutex_init(&vm->eviction_lock); vm->evicting = false; diff --git a/drivers/gpu/drm/drm_syncobj.c b/drivers/gpu/drm/drm_syncobj.c index 7e48dcd1bee4..5a961ea90a35 100644 --- a/drivers/gpu/drm/drm_syncobj.c +++ b/drivers/gpu/drm/drm_syncobj.c @@ -255,7 +255,7 @@ static void drm_syncobj_fence_add_wait(struct drm_syncobj *syncobj, dma_fence_put(fence); list_add_tail(&wait->node, &syncobj->cb_list); } else if (!fence) {

• wait->fence = dma_fence_get_stub();
  

• wait->fence = dma_fence_get_stub(false);
  

  } else { wait->fence = fence; }

@@ -411,7 +411,7 @@ int drm_syncobj_find_fence(struct drm_file *file_private, * signalled, use a new fence instead. */ if (!*fence)

• 		*fence = dma_fence_get_stub();
  

• 		*fence = dma_fence_get_stub(false);
  

goto out; } @@ -1000,7 +1000,7 @@ static void syncobj_wait_syncobj_func(struct drm_syncobj *syncobj, dma_fence_put(fence); return; } else if (!fence) {

• wait->fence = dma_fence_get_stub();
  

• wait->fence = dma_fence_get_stub(false);
  

  } else { wait->fence = fence; }

@@ -1067,7 +1067,7 @@ static signed long drm_syncobj_array_wait_timeout(struct drm_syncobj **syncobjs, if (fence) entries[i].fence = fence; else

• 	entries[i].fence = dma_fence_get_stub();
  

• 	entries[i].fence = dma_fence_get_stub(false);
  

if ((flags & DRM_SYNCOBJ_WAIT_FLAGS_WAIT_AVAILABLE) || dma_fence_is_signaled(entries[i].fence)) { @@ -1472,7 +1472,7 @@ drm_syncobj_timeline_signal_ioctl(struct drm_device *dev, void *data, } for (i = 0; i < args->count_handles; i++) {

• struct dma_fence *fence = dma_fence_get_stub();
  

• struct dma_fence *fence = dma_fence_get_stub(false);
  

drm_syncobj_add_point(syncobjs[i], chains[i], fence, points[i]); diff --git a/include/linux/dma-fence.h b/include/linux/dma-fence.h index afea82ec5946..be96687d31d8 100644 --- a/include/linux/dma-fence.h +++ b/include/linux/dma-fence.h @@ -99,6 +99,16 @@ enum dma_fence_flag_bits { DMA_FENCE_FLAG_SIGNALED_BIT, DMA_FENCE_FLAG_TIMESTAMP_BIT, DMA_FENCE_FLAG_ENABLE_SIGNAL_BIT,

• /**

• * @DMA_FENCE_FLAG_USER:
  

• *
  

• * Indicates an user fence. User fences are not guaranteed to signal in
  

• * a finite amount of time. Because of this it is not allowed to wait for user
  

• * fences with any lock held nor depend the signaling of none user
  

• * fences on them.
  

• */
  

• DMA_FENCE_FLAG_USER, DMA_FENCE_FLAG_DRIVER, /* must always be last member */ };

@@ -398,6 +408,8 @@ void dma_fence_enable_sw_signaling(struct dma_fence *fence); static inline bool dma_fence_is_signaled_locked(struct dma_fence *fence) {

• WARN_ON(test_bit(DMA_FENCE_FLAG_USER, &fence->flags));
• if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) return true;

@@ -428,6 +440,9 @@ dma_fence_is_signaled_locked(struct dma_fence *fence) static inline bool dma_fence_is_signaled(struct dma_fence *fence) {

• if (test_bit(DMA_FENCE_FLAG_USER, &fence->flags))

• might_fault();
  

Why this one can fault and the dma_fence_signal_locked cannot? I mean obviosuly it must not in the locked section, but how do signalling callbacks know the context of the caller?

This maybe ties back to the issue I don't think I ever found an explanation to - why "is signal" helpers actually need (or want?) to do the signalling itself, and are therefore hit also with the signalling annotations?

Regards,

Tvrtko

• if (test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags)) return true;

@@ -583,7 +598,7 @@ static inline signed long dma_fence_wait(struct dma_fence *fence, bool intr) return ret < 0 ? ret : 0; } -struct dma_fence *dma_fence_get_stub(void); +struct dma_fence *dma_fence_get_stub(bool user); struct dma_fence *dma_fence_allocate_private_stub(void); u64 dma_fence_context_alloc(unsigned num);