Re: [PATCH 1/2] dma-buf: Assign separate lockdep class to chain lock

[Boris Brezillon <boris.brezillon@collabora.com>]

On Tue, 24 Feb 2026 09:55:43 -0800
Matthew Brost <matthew.brost@intel.com> wrote:

> dma_fence_chain_enable_signaling() runs while holding the chain
> inline_lock and may add callbacks to underlying fences, which takes
> their inline_lock.
> 
> Since both locks share the same lockdep class, this valid nesting
> triggers a recursive locking warning. Assign a distinct lockdep class
> to the chain inline_lock so lockdep can correctly model the hierarchy.
> 
> Fixes: a408c0ca0c41 ("dma-buf: use inline lock for the dma-fence-chain")
> Cc: Christian König <christian.koenig@amd.com>
> Cc: Tvrtko Ursulin <tvrtko.ursulin@igalia.com>
> Cc: Philipp Stanner <phasta@kernel.org>
> Cc: Boris Brezillon <boris.brezillon@collabora.com>
> Signed-off-by: Matthew Brost <matthew.brost@intel.com>
> ---
>  drivers/dma-buf/dma-fence-chain.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/drivers/dma-buf/dma-fence-chain.c b/drivers/dma-buf/dma-fence-chain.c
> index a707792b6025..4c2a9f2ce126 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_LOCKDEP)
> +	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 +266,20 @@ 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_LOCKDEP)
> +	/*
> +	 * dma_fence_chain_enable_signaling() is invoked while holding
> +	 * chain->base.inline_lock and may call dma_fence_add_callback()
> +	 * on the underlying fences, which takes their inline_lock.
> +	 *
> +	 * Since both locks share the same lockdep class, this legitimate
> +	 * nesting confuses lockdep and triggers a recursive locking
> +	 * warning. Assign a separate lockdep class to the chain lock
> +	 * to model this hierarchy correctly.
> +	 */
> +	lockdep_set_class(&chain->base.inline_lock, &dma_fence_chain_lock_key);
> +#endif

If we're going to recommend the use of this inline_lock for all new
dma_fence_ops implementers, as the commit message seems to imply

> Shared spinlocks have the problem that implementations need to guarantee
> that the lock lives at least as long all fences referencing them.
> 
> Using a per-fence spinlock allows completely decoupling spinlock
> producer and consumer life times, simplifying the handling in most use
> cases.

maybe we should have the lock_class_key at the dma_buf_ops level and
have this lockdep_set_class() automated in __dma_fence_init().

> +
>  	/*
>  	 * Chaining dma_fence_chain container together is only allowed through
>  	 * the prev fence and not through the contained fence.