Re: [PATCH] nouveau/fence: handle cross device fences properly.

[Ben Skeggs <bskeggs@nvidia.com>]

On 7/1/25 15:58, Dave Airlie wrote:

> From: Dave Airlie <airlied@redhat.com>
>
> If we have two nouveau controlled devices and one passes a dma-fence
> to the other, when we hit the sync path it can cause the second device
> to try and put a sync wait in it's pushbuf for the seqno of the context
> on the first device.
>
> Since fence contexts are vmm bound, check the if vmm's match between
> both users, this should ensure that fence seqnos don't get used wrongly
> on incorrect channels.
>
> This seems to happen fairly spuriously and I found it tracking down
> a multi-card regression report, that seems to work by luck before this.
>
> Signed-off-by: Dave Airlie <airlied@redhat.com>
> Cc: stable@vger.kernel.org

Reviewed-by: Ben Skeggs <bskeggs@nvidia.com>


> ---
>   drivers/gpu/drm/nouveau/nouveau_fence.c | 3 ++-
>   1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/gpu/drm/nouveau/nouveau_fence.c b/drivers/gpu/drm/nouveau/nouveau_fence.c
> index ee5e9d40c166f..5743c82f4094b 100644
> --- a/drivers/gpu/drm/nouveau/nouveau_fence.c
> +++ b/drivers/gpu/drm/nouveau/nouveau_fence.c
> @@ -370,7 +370,8 @@ nouveau_fence_sync(struct nouveau_bo *nvbo, struct nouveau_channel *chan,
>   
>   				rcu_read_lock();
>   				prev = rcu_dereference(f->channel);
> -				if (prev && (prev == chan ||
> +				if (prev && (prev->vmm == chan->vmm) &&
> +				    (prev == chan ||
>   					     fctx->sync(f, prev, chan) == 0))
>   					must_wait = false;
>   				rcu_read_unlock();