Re: [PATCH V13 2/7] dma-resv: Fix undefined symbol when CONFIG_DMA_SHARED_BUFFER is disabled

Christian König
2026-04-24T07:56:31Z

On 4/24/26 07:47, Mukesh Kumar Chaurasiya (IBM) wrote:
> When building with LLVM=1 for architectures like powerpc where
> CONFIG_DMA_SHARED_BUFFER is not enabled, the build fails with:
>
> ld.lld: error: undefined symbol: dma_resv_reset_max_fences
> >>> referenced by helpers.c
> >>> rust/helpers/helpers.o:(rust_helper_dma_resv_unlock)
>
> The issue occurs because:
> 1. CONFIG_DEBUG_MUTEXES=y is enabled
> 2. CONFIG_DMA_SHARED_BUFFER is not enabled
> 3. dma_resv_reset_max_fences() is declared in the header when
> CONFIG_DEBUG_MUTEXES is set
> 4. But the function is only compiled in drivers/dma-buf/dma-resv.c,
> which is only built when CONFIG_DMA_SHARED_BUFFER is enabled
> 5. Rust helpers call dma_resv_unlock() which calls
> dma_resv_reset_max_fences(), causing an undefined symbol
>
> Fix this by making the function declaration conditional on both
> CONFIG_DEBUG_MUTEXES and CONFIG_DMA_SHARED_BUFFER. When either is
> disabled, use a static inline stub instead.

Well we are clearly missing something here, but of hand that doesn't looks like the right fix.

When CONFIG_DMA_SHARED_BUFFER isn't enabled then the whole dma-resv.h header can't be used at all.

So you also can't call dma_resv_unlock() from the Rust helpers. Which means that we need to make the Rust helpers somehow depend on CONFIG_DMA_SHARED_BUFFER.

Alternative would be to provide dummies for the functions in dma-resv.h when CONFIG_DMA_SHARED_BUFFER isn't set, but that looks a bit like it just hides the issue.

Regards,
Christian.

>
> Fixes: 0c6b522abc2a ("dma-buf: cleanup dma-resv shared fence debugging a bit v2")
> Signed-off-by: Mukesh Kumar Chaurasiya (IBM) <mkchauras@gmail.com>
> ---
> include/linux/dma-resv.h | 2 +-
> 1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/include/linux/dma-resv.h b/include/linux/dma-resv.h
> index c5ab6fd9ebe8..23c8db0b5214 100644
> --- a/include/linux/dma-resv.h
> +++ b/include/linux/dma-resv.h
> @@ -311,7 +311,7 @@ static inline bool dma_resv_iter_is_restarted(struct dma_resv_iter *cursor)
> #define dma_resv_held(obj) lockdep_is_held(&(obj)->lock.base)
> #define dma_resv_assert_held(obj) lockdep_assert_held(&(obj)->lock.base)
>
> -#ifdef CONFIG_DEBUG_MUTEXES
> +#if IS_ENABLED(CONFIG_DEBUG_MUTEXES) && IS_ENABLED(CONFIG_DMA_SHARED_BUFFER)
> void dma_resv_reset_max_fences(struct dma_resv *obj);
> #else
> static inline void dma_resv_reset_max_fences(struct dma_resv *obj) {}
> --
> 2.53.0
>