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

Mukesh Kumar Chaurasiya
2026-04-24T08:34:28Z

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"

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
> >>>
> >>
>