Re: [PATCH 03/15] dma-buf: protected fence ops by RCU

[Tvrtko Ursulin]

One additional comment on this patch:

On 13/10/2025 14:48, Christian König wrote:
At first glance it is counter intuitive to protect a constant function
pointer table by RCU, but this allows modules providing the function
table to unload by waiting for an RCU grace period.

Signed-off-by: Christian König <christian.koe...@amd.com>
---
  drivers/dma-buf/dma-fence.c | 65 +++++++++++++++++++++++++++----------
  include/linux/dma-fence.h   | 18 ++++++++--
  2 files changed, 62 insertions(+), 21 deletions(-)


8><

@@ -1104,11 +1127,14 @@ EXPORT_SYMBOL(dma_fence_init64);
   */
  const char __rcu *dma_fence_driver_name(struct dma_fence *fence)
  {
+       const struct dma_fence_ops *ops;
+
        RCU_LOCKDEP_WARN(!rcu_read_lock_held(),
                         "RCU protection is required for safe access to returned 
string");
  
+	ops = rcu_dereference(fence->ops);
        if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
-               return fence->ops->get_driver_name(fence);
+               return ops->get_driver_name(fence);
        else
                return "detached-driver";
  }
@@ -1136,11 +1162,14 @@ EXPORT_SYMBOL(dma_fence_driver_name);
   */
  const char __rcu *dma_fence_timeline_name(struct dma_fence *fence)
  {
+       const struct dma_fence_ops *ops;
+
        RCU_LOCKDEP_WARN(!rcu_read_lock_held(),
                         "RCU protection is required for safe access to returned 
string");
  
+	ops = rcu_dereference(fence->ops);

For the above two functions, the RCU_LOCKDEP_WARN now becomes redundant 
to the one rcu_dererence() would emit. Maybe just move the string into a 
comment?

Regards,

Tvrtko

        if (!test_bit(DMA_FENCE_FLAG_SIGNALED_BIT, &fence->flags))
-               return fence->ops->get_driver_name(fence);
+               return ops->get_driver_name(fence);
        else
                return "signaled-timeline";
  }