[Linaro-mm-sig] Re: [PATCH] drm/nouveau: Prevent signalled fences in pending list

Am 27.03.25 um 09:42 schrieb Philipp Stanner:

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 the fence.

Good catch.

Move the code from nouveau_fence_signal() into a dma_fence callback. Register that callback when creating the fence.

But that's a really ugly approach.

Either nouveau shouldn't implement the signaled callback or make sure that when returning true from the signaled callback the fence is also removed from the pending list.

Cc: stable@vger.kernel.org # 4.10+ Fixes: f54d1867005c ("dma-buf: Rename struct fence to dma_fence") Signed-off-by: Philipp Stanner phasta@kernel.org

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I'm not entirely sure what Fixes-Tag is appropriate. The last time the line causing the signalled fence in the list was touched is the commit listed above.

Yeah, that's most likely not correct. My educated guess is that the bug was always there just never discovered.

Regards, Christian.

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drivers/gpu/drm/nouveau/nouveau_fence.c | 41 ++++++++++++++++--------- drivers/gpu/drm/nouveau/nouveau_fence.h | 1 + 2 files changed, 27 insertions(+), 15 deletions(-)

diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c b/drivers/gpu/drm/nouveau/nouveau_fence.c index 7cc84472cece..b2c2241a8803 100644 --- a/drivers/gpu/drm/nouveau/nouveau_fence.c +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c @@ -50,24 +50,22 @@ 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;
  

• }

• if (test_bit(DMA_FENCE_FLAG_USER_BITS, &fence->base.flags))

• --fctx->notify_ref;
  

dma_fence_put(&fence->base);

• return drop;

} static struct nouveau_fence * @@ -93,7 +91,8 @@ 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))
  

• dma_fence_signal_locked(&fence->base);
  

• if (fctx->notify_ref == 0)
  nvif_event_block(&fctx->event);
  

  } fctx->killed = 1;

@@ -131,7 +130,6 @@ static int 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 +138,10 @@ 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;

• return fctx->notify_ref == 0 ? 1 : 0;

} static void @@ -235,6 +233,19 @@ 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);

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;