Re: [PATCH v2] drm/nouveau: Prevent signalled fences in pending list

Philipp Stanner
2025-04-03T10:17:36Z

On Thu, 2025-04-03 at 12:13 +0200, Philipp Stanner wrote:
> Nouveau currently relies on the assumption that dma_fences will only
> ever get signalled through nouveau_fence_signal(), which takes care
> of
> removing a signalled fence from the list nouveau_fence_chan.pending.
>
> This self-imposed rule is violated in nouveau_fence_done(), where
> dma_fence_is_signaled() can signal the fence without removing it from
> the list. This enables accesses to already signalled fences through
> the
> list, which is a bug.
>
> Furthermore, it must always be possible to use standard dma_fence
> methods an a dma_fence and observe valid behavior. The canonical way
> of
> ensuring that signalling a fence has additional effects is to add
> those
> effects to a callback and register it on that fence.
>
> Move the code from nouveau_fence_signal() into a dma_fence callback.
> Register that callback when creating the fence.
>
> Cc: <stable@vger.kernel.org> # 4.10+
> Signed-off-by: Philipp Stanner <phasta@kernel.org>
> ---
> Changes in v2:
>   - Remove Fixes: tag. (Danilo)
>   - Remove integer "drop" and call nvif_event_block() in the fence
>     callback. (Danilo)
> ---
>  drivers/gpu/drm/nouveau/nouveau_fence.c | 52 +++++++++++++----------
> --
>  drivers/gpu/drm/nouveau/nouveau_fence.h |  1 +
>  2 files changed, 29 insertions(+), 24 deletions(-)
>
> diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c
> b/drivers/gpu/drm/nouveau/nouveau_fence.c
> index 7cc84472cece..cf510ef9641a 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_fence.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
> @@ -50,24 +50,24 @@ nouveau_fctx(struct nouveau_fence *fence)
>   return container_of(fence->base.lock, struct
> nouveau_fence_chan, lock);
>  }
>
> -static int
> -nouveau_fence_signal(struct nouveau_fence *fence)
> +static void
> +nouveau_fence_cleanup_cb(struct dma_fence *dfence, struct
> dma_fence_cb *cb)
>  {
> - int drop = 0;
> + struct nouveau_fence_chan *fctx;
> + struct nouveau_fence *fence;
> +
> + fence = container_of(dfence, struct nouveau_fence, base);
> + fctx = nouveau_fctx(fence);
>
> - dma_fence_signal_locked(&fence->base);
>   list_del(&fence->head);
>   rcu_assign_pointer(fence->channel, NULL);
>
>   if (test_bit(DMA_FENCE_FLAG_USER_BITS, &fence->base.flags))
> {
> - struct nouveau_fence_chan *fctx =
> nouveau_fctx(fence);
> -
>   if (!--fctx->notify_ref)
> - drop = 1;
> + nvif_event_block(&fctx->event);
>   }
>
>   dma_fence_put(&fence->base);

What I realized while coding this v2 is that we might want to think
about whether we really want the dma_fence_put() in the fence callback?

It should work fine, since it's exactly identical to the previous
code's behavior – but effectively it means that the driver's reference
will be dropped whenever it signals that fence.

IDK

P.

> - return drop;
>  }
>
>  static struct nouveau_fence *
> @@ -93,8 +93,7 @@ nouveau_fence_context_kill(struct
> nouveau_fence_chan *fctx, int error)
>   if (error)
>   dma_fence_set_error(&fence->base, error);
>
> - if (nouveau_fence_signal(fence))
> - nvif_event_block(&fctx->event);
> + dma_fence_signal_locked(&fence->base);
>   }
>   fctx->killed = 1;
>   spin_unlock_irqrestore(&fctx->lock, flags);
> @@ -127,11 +126,10 @@ nouveau_fence_context_free(struct
> nouveau_fence_chan *fctx)
>   kref_put(&fctx->fence_ref, nouveau_fence_context_put);
>  }
>
> -static int
> +static void
>  nouveau_fence_update(struct nouveau_channel *chan, struct
> nouveau_fence_chan *fctx)
>  {
>   struct nouveau_fence *fence;
> - int drop = 0;
>   u32 seq = fctx->read(chan);
>
>   while (!list_empty(&fctx->pending)) {
> @@ -140,10 +138,8 @@ nouveau_fence_update(struct nouveau_channel
> *chan, struct nouveau_fence_chan *fc
>   if ((int)(seq - fence->base.seqno) < 0)
>   break;
>
> - drop |= nouveau_fence_signal(fence);
> + dma_fence_signal_locked(&fence->base);
>   }
> -
> - return drop;
>  }
>
>  static void
> @@ -152,7 +148,6 @@ nouveau_fence_uevent_work(struct work_struct
> *work)
>   struct nouveau_fence_chan *fctx = container_of(work, struct
> nouveau_fence_chan,
>          uevent_work);
>   unsigned long flags;
> - int drop = 0;
>
>   spin_lock_irqsave(&fctx->lock, flags);
>   if (!list_empty(&fctx->pending)) {
> @@ -161,11 +156,8 @@ nouveau_fence_uevent_work(struct work_struct
> *work)
>
>   fence = list_entry(fctx->pending.next,
> typeof(*fence), head);
>   chan = rcu_dereference_protected(fence->channel,
> lockdep_is_held(&fctx->lock));
> - if (nouveau_fence_update(chan, fctx))
> - drop = 1;
> + nouveau_fence_update(chan, fctx);
>   }
> - if (drop)
> - nvif_event_block(&fctx->event);
>
>   spin_unlock_irqrestore(&fctx->lock, flags);
>  }
> @@ -235,6 +227,19 @@ nouveau_fence_emit(struct nouveau_fence *fence)
>          &fctx->lock, fctx->context, ++fctx-
> >sequence);
>   kref_get(&fctx->fence_ref);
>
> + fence->cb.func = nouveau_fence_cleanup_cb;
> + /* Adding a callback runs into
> __dma_fence_enable_signaling(), which will
> + * ultimately run into nouveau_fence_no_signaling(), where a
> WARN_ON
> + * would fire because the refcount can be dropped there.
> + *
> + * Increment the refcount here temporarily to work around
> that.
> + */
> + dma_fence_get(&fence->base);
> + ret = dma_fence_add_callback(&fence->base, &fence->cb,
> nouveau_fence_cleanup_cb);
> + dma_fence_put(&fence->base);
> + if (ret)
> + return ret;
> +
>   ret = fctx->emit(fence);
>   if (!ret) {
>   dma_fence_get(&fence->base);
> @@ -246,8 +251,7 @@ nouveau_fence_emit(struct nouveau_fence *fence)
>   return -ENODEV;
>   }
>
> - if (nouveau_fence_update(chan, fctx))
> - nvif_event_block(&fctx->event);
> + nouveau_fence_update(chan, fctx);
>
>   list_add_tail(&fence->head, &fctx->pending);
>   spin_unlock_irq(&fctx->lock);
> @@ -270,8 +274,8 @@ nouveau_fence_done(struct nouveau_fence *fence)
>
>   spin_lock_irqsave(&fctx->lock, flags);
>   chan = rcu_dereference_protected(fence->channel,
> lockdep_is_held(&fctx->lock));
> - if (chan && nouveau_fence_update(chan, fctx))
> - nvif_event_block(&fctx->event);
> + if (chan)
> + nouveau_fence_update(chan, fctx);
>   spin_unlock_irqrestore(&fctx->lock, flags);
>   }
>   return dma_fence_is_signaled(&fence->base);
> diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.h
> b/drivers/gpu/drm/nouveau/nouveau_fence.h
> index 8bc065acfe35..e6b2df7fdc42 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_fence.h
> +++ b/drivers/gpu/drm/nouveau/nouveau_fence.h
> @@ -10,6 +10,7 @@ struct nouveau_bo;
>
>  struct nouveau_fence {
>   struct dma_fence base;
> + struct dma_fence_cb cb;
>
>   struct list_head head;
>