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

Christian König
2026-04-24T08:45:03Z

On 4/24/26 10:34, Mukesh Kumar Chaurasiya wrote:
> On Fri, Apr 24, 2026 at 10:25:25AM +0200, Christian König wrote:
>> On 4/24/26 10:06, Mukesh Kumar Chaurasiya wrote:
>>> [Sie erhalten nicht häufig E-Mails von mkchauras@gmail.com. Weitere Informationen, warum dies wichtig ist, finden Sie unter https://aka.ms/LearnAboutSenderIdentification ]
>>>
>>> On Fri, Apr 24, 2026 at 09:56:16AM +0200, Christian König wrote:
>>>> 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.
>>>>
>>> What about something like this:
>>>
>>> diff --git a/rust/helpers/dma-resv.c b/rust/helpers/dma-resv.c
>>> index 71914d8241e2..53c119f1b144 100644
>>> --- a/rust/helpers/dma-resv.c
>>> +++ b/rust/helpers/dma-resv.c
>>> @@ -2,6 +2,7 @@
>>>
>>> #include <linux/dma-resv.h>
>>>
>>> +#ifdef CONFIG_DMA_SHARED_BUFFER
>>> __rust_helper
>>> int rust_helper_dma_resv_lock(struct dma_resv *obj, struct ww_acquire_ctx *ctx)
>>> {
>>> @@ -12,3 +13,4 @@ __rust_helper void rust_helper_dma_resv_unlock(struct dma_resv *obj)
>>> {
>>> dma_resv_unlock(obj);
>>> }
>>> +#endif
>>>
>>> This seems to fix the issue and makes sense, whoever wants to use the
>>> dma shared buffer will anyway enable the config
>>
>> Yeah that directions makes sense.
>>
>> I would maybe go a step further and don't even compile rust/helpers/dma-resv.c when CONFIG_DMA_SHARED_BUFFER isn't set.
>>
> This makes sense.
>> But if that is not possible for some reason then this solution is perfectly fine with me as well.
>>
>> The general goal is to make the error messages when you haven't set CONFIG_DMA_SHARED_BUFFER and still try to use the DMA-buf functions easy to understand and not something cryptic.
>>
>> Thanks,
>> Christian.
>>
>
> What about this one?
>
> diff --git a/rust/helpers/helpers.c b/rust/helpers/helpers.c
> index 625921e27dfb..09ee5cac600d 100644
> --- a/rust/helpers/helpers.c
> +++ b/rust/helpers/helpers.c
> @@ -57,7 +57,9 @@
> #include "cred.c"
> #include "device.c"
> #include "dma.c"
> +#ifdef CONFIG_DMA_SHARED_BUFFER
> #include "dma-resv.c"
> +#endif
> #include "drm.c"
> #include "err.c"
> #include "irq.c"

Looks good, feel free to add Reviewed-by: Christian König <christian.koenig@amd.com>

But somebody more familiar with the Rust helpers should take another look on it.

Regards,
Christian.

>
> Regards,
> Mukesh
>
>>>
>>> Regards,
>>> Mukesh
>>>>>
>>>>> 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
>>>>>
>>>>
>>