Re: [PATCH] drm/drm_crtc: fix race with dma_fence_signal() in ::get_driver_name()

[Philipp Stanner]

+Cc Danilo

On Thu, 2026-06-18 at 15:03 +0100, André Draszik wrote:
> Since commit 541c8f2468b9 ("dma-buf: detach fence ops on signal v3"),
> I'm seeing the BUG_ON() triggering in drm_crtc's fence_to_crtc() via
> drm_crtc_fence_get_driver_name() regularly:
> 
>     Call trace:
>      panic+0x58/0x5c
>      die+0x160/0x178
>      bug_brk_handler+0x70/0xa4
>      call_el1_break_hook+0x3c/0x1a0
>      do_el1_brk64+0x24/0x74
>      el1_brk64+0x34/0x54
>      el1h_64_sync_handler+0x80/0xfc
>      el1h_64_sync+0x84/0x88
>      drm_crtc_fence_get_driver_name+0x60/0x68 (P)
>      sync_file_get_name+0x184/0x45c
>      sync_file_ioctl+0x404/0xf70
>      __arm64_sys_ioctl+0x124/0x1dc
> 
> This looks to be caused by a code flow similar to the following:
> 
> +++ snip +++
> thread A                             thread B
> 
>                                      ioctl(SYNC_IOC_FILE_INFO)
>                                      sync_file_ioctl()
>                                      sync_file_get_name()
> dma_fence_signal_timestamp_locked()  dma_fence_driver_name()
>                                        ops = rcu_dereference(fence->ops)
>                                        if (!dma_fence_test_signaled_flag())
>                                          ops->get_driver_name(fence) i.e.
>                                          drm_crtc_fence_get_driver_name()
> test_and_set_bit(SIGNALED)
> RCU_INIT_POINTER(fence->ops, NULL)
>                                      drm_crtc_fence_get_driver_name()
>                                        BUG_ON(rcu_access_pointer(fence->ops)
>                                               != &drm_crtc_fence_ops)

Now this looks like a very similar problem that I have recently been
concerned with:

https://lore.kernel.org/dri-devel/20260612104251.2264707-2-phasta@xxxxxxxxxx/

https://lore.kernel.org/dri-devel/fa0dc9757bf8343516c4b156a2b70ec91b64ef8f.camel@xxxxxxxxxxx/


I continue to believe because of bugs like this and the ones I have
quoted in the threads above the robustness of the kernel could be
greatly improved if we could get dma_fence fully synchronized with its
lock.


That said:

> +++ snap +++
> 
> I see two ways to resolve this:
> a) simply drop the BUG_ON(). It can not work anymore since above
>    commit, as it is racy now.
> b) pass the original 'ops' pointer obtained in dma_fence_driver_name()
>    to all callees.
> 
> This patch implements option a), as because:
> * I don't see much benefit in passing the extra pointer just for this
>   BUG_ON() to work.
> * Requiring the dma_fence_ops in those callbacks is an implementation
>   detail of the drm_crtc driver, and therefore upper layers shouldn't
>   have to care about that.
> * The existence of the BUG_ON() doesn't appear to be consistent with
>   implementations of ::get_driver_name() or ::get_timeline_name() in
>   the majority of other DRM drivers in the first place. Those that do
>   have a similar BUG_ON() (i915, xe) probably also need an update
>   similar to this patch here but I'm not in a position to test those.
> 
> Note that the adjacent drm_crtc_fence_get_timeline_name() has the same
> problem and is fixed by this patch as well.
> 
> Fixes: 541c8f2468b9 ("dma-buf: detach fence ops on signal v3")
> Signed-off-by: André Draszik <andre.draszik@xxxxxxxxxx>
> ---
>  drivers/gpu/drm/drm_crtc.c | 11 +++--------
>  1 file changed, 3 insertions(+), 8 deletions(-)
> 
> diff --git a/drivers/gpu/drm/drm_crtc.c b/drivers/gpu/drm/drm_crtc.c
> index 63ead8ba6756..31c8636e7467 100644
> --- a/drivers/gpu/drm/drm_crtc.c
> +++ b/drivers/gpu/drm/drm_crtc.c
> @@ -73,6 +73,9 @@
>   * &drm_mode_config_funcs.atomic_check.
>   */
>  
> +#define fence_to_crtc(f) container_of((f)->extern_lock, \
> +				      struct drm_crtc, fence_lock)

I agree that macros should be avoided if possible.

> +
>  /**
>   * drm_crtc_from_index - find the registered CRTC at an index
>   * @dev: DRM device
> @@ -154,14 +157,6 @@ static void drm_crtc_crc_fini(struct drm_crtc *crtc)
>  #endif
>  }
>  
> -static const struct dma_fence_ops drm_crtc_fence_ops;
> -
> -static struct drm_crtc *fence_to_crtc(struct dma_fence *fence)
> -{
> -	BUG_ON(rcu_access_pointer(fence->ops) != &drm_crtc_fence_ops);

+1

BUG_ON is more or less deprecated and should not be used anymore. There
needs to be bombastic justification for shooting down the entire
kernel.


P.

> -	return container_of(fence->extern_lock, struct drm_crtc, fence_lock);
> -}
> -
>  static const char *drm_crtc_fence_get_driver_name(struct dma_fence *fence)
>  {
>  	struct drm_crtc *crtc = fence_to_crtc(fence);
> 
> ---
> base-commit: e2cae00c05d196491c318196792297f2dfbaa02c
> change-id: 20260618-linux-drm_crtc_fix2-23a7c354a412
> 
> Best regards,