Re: [PATCH] dma-buf: revert "use inline lock for the dma-fence-chain"

[Matthew Brost <matthew.brost@intel.com>]

On Mon, Feb 23, 2026 at 08:57:05PM +0100, Christian König wrote:
> This reverts commit a408c0ca0c411ca1ead995bdae3112a806c87556.
> 
> This causes a lockdep splat. Not really the right fix, but changing this
> is more work than expected.
> 

I think dma-fence-arrays will have the same issue.

Can’t we just assign a lockdep key for chains and arrays?

Typing as fast as I could, I came up with this, and it seems to work:

diff --git a/drivers/dma-buf/dma-fence-array.c b/drivers/dma-buf/dma-fence-array.c
index cd970eceaefb..d5cce24dca5b 100644
--- a/drivers/dma-buf/dma-fence-array.c
+++ b/drivers/dma-buf/dma-fence-array.c
@@ -200,6 +200,10 @@ void dma_fence_array_init(struct dma_fence_array *array,
                          u64 context, unsigned seqno,
                          bool signal_on_any)
 {
+#if IS_ENABLED(CONFIG_PROVE_LOCKING)
+       static struct lock_class_key dma_fence_array_lock_key;
+#endif
+
        WARN_ON(!num_fences || !fences);

        array->num_fences = num_fences;
@@ -208,6 +212,10 @@ void dma_fence_array_init(struct dma_fence_array *array,
                       seqno);
        init_irq_work(&array->work, irq_dma_fence_array_work);

+#if IS_ENABLED(CONFIG_PROVE_LOCKING)
+       lockdep_set_class(&array->base.inline_lock, &dma_fence_array_lock_key);
+#endif
+
        atomic_set(&array->num_pending, signal_on_any ? 1 : num_fences);
        array->fences = fences;

diff --git a/drivers/dma-buf/dma-fence-chain.c b/drivers/dma-buf/dma-fence-chain.c
index a707792b6025..aa144b8c3534 100644
--- a/drivers/dma-buf/dma-fence-chain.c
+++ b/drivers/dma-buf/dma-fence-chain.c
@@ -242,6 +242,9 @@ void dma_fence_chain_init(struct dma_fence_chain *chain,
                          struct dma_fence *fence,
                          uint64_t seqno)
 {
+#if IS_ENABLED(CONFIG_PROVE_LOCKING)
+       static struct lock_class_key dma_fence_chain_lock_key;
+#endif
        struct dma_fence_chain *prev_chain = to_dma_fence_chain(prev);
        uint64_t context;

@@ -263,6 +266,10 @@ void dma_fence_chain_init(struct dma_fence_chain *chain,
        dma_fence_init64(&chain->base, &dma_fence_chain_ops, NULL,
                         context, seqno);

+#if IS_ENABLED(CONFIG_PROVE_LOCKING)
+       lockdep_set_class(&chain->base.inline_lock, &dma_fence_chain_lock_key);
+#endif
+
        /*
         * Chaining dma_fence_chain container together is only allowed through
         * the prev fence and not through the contained fence.

Matt

> Signed-off-by: Christian König <christian.koenig@amd.com>
> ---
>  drivers/dma-buf/dma-fence-chain.c | 3 ++-
>  include/linux/dma-fence-chain.h   | 1 +
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/dma-buf/dma-fence-chain.c b/drivers/dma-buf/dma-fence-chain.c
> index a707792b6025..a8a90acf4f34 100644
> --- a/drivers/dma-buf/dma-fence-chain.c
> +++ b/drivers/dma-buf/dma-fence-chain.c
> @@ -245,6 +245,7 @@ void dma_fence_chain_init(struct dma_fence_chain *chain,
>  	struct dma_fence_chain *prev_chain = to_dma_fence_chain(prev);
>  	uint64_t context;
>  
> +	spin_lock_init(&chain->lock);
>  	rcu_assign_pointer(chain->prev, prev);
>  	chain->fence = fence;
>  	chain->prev_seqno = 0;
> @@ -260,7 +261,7 @@ void dma_fence_chain_init(struct dma_fence_chain *chain,
>  			seqno = max(prev->seqno, seqno);
>  	}
>  
> -	dma_fence_init64(&chain->base, &dma_fence_chain_ops, NULL,
> +	dma_fence_init64(&chain->base, &dma_fence_chain_ops, &chain->lock,
>  			 context, seqno);
>  
>  	/*
> diff --git a/include/linux/dma-fence-chain.h b/include/linux/dma-fence-chain.h
> index df3beadf1515..5cd3ba53b4a1 100644
> --- a/include/linux/dma-fence-chain.h
> +++ b/include/linux/dma-fence-chain.h
> @@ -46,6 +46,7 @@ struct dma_fence_chain {
>  		 */
>  		struct irq_work work;
>  	};
> +	spinlock_t lock;
>  };
>  
>  
> -- 
> 2.43.0
>